The Rebel Alliance from Star Wars blowing up not only the first Death Star, but the second one as well.
The destruction of Vulcan in the 2009 Star Trek. Not only was a massive loss for the Federation, but a signal to the fans that this was not going back to the status quo.
The Battle Of Yonkers. They threw everything but the kitchen sink at the enemy and still lost. Though that was actually a detriment in the end, as all their gear and defenses and tactics were based on fighting an enemy that obeyed no human nor life norms (no pain, no fear, no stopping...)
The Fall of Coruscant to the Yuzzhan Vong in the New Jedi Order series. Under the incredulous eyes of many of the surviving characters, the lights of The City That Never Sleeps go out for the first time in several thousand years.
The Dropsite Massacre of Isstvan V from the Horus Heresy. Four noble space marine legions had suddenly turned traitor and fortified themselves on said planet (after burning Isstvan III and purging their own ranks of traitors), an overwhelming seven legions were sent to crush the rebellion before it could spread. Instead, four of those legions turned traitor too, and all of them caught the remaining three loyalist legions in a crossfire that saw hundreds of thousands dead. It went From Bad to Worse from there...
The Silmarillion: The Battle Of Unnumbered Tears. It begins as a noble effort of the Elves, Men, and Dwarves to finish Morgoth once and for all. It's the first coalition of all the races together to fight Morgoth, and the greatest army seen so far in the world outside of the gods. It gets crushed so badly and so many people die that Morgoth literally makes a hill out of the corpses. The worst part is that they never had a chance. And things get so much worse from there.
In Honor Harrington these were the Battle of Manticore for Haven and Grendelsbane disaster during the opening stages of the Second Havenite War and later the Operation Oyster Bay (though Manticorans didn't knew its official name) for Manticore, though both nations recovered from these pretty damn quickly, and with a vengeance.
Andrey Livadny's The History of the Galaxy series has the nuclear bombardment of the Dabog colony by the Earth Alliance strike fleet as the opening round of the First Galactic War. In fact, this trope was President Hammer's original plan. The fleet would show up without warning, nuke two major cities on the planet, and land troops to quickly rout any defenders. Unfortunately for them, they didn't expect the colonists to have very effective Real Robots that routed the invaders. The pissed-off admiral of the fleet has the planet nuked so thoroughly that 1000 years later it's still uninhabitable. In fact, even after a millenium, the mere mention of Dabog is enough to stop a fight. However, the destruction of the colony only serves to band the colonies together and, after decades of vicious fighting, take the fight to Earth itself.
In Codex Alera the rampant destruction of the Vord War, epitomised by the obliteration of Alera Imperia, the capital city, and the death of Gaius Sextus, the First Lord, is certain to be seen as this in the future (though the end of the series means readers won't see it). However, while the Vord War was the worst disaster in Aleran history in terms of casualties and destruction, various characters think that the Aleran culture is likely to improve a great deal in the aftermath, as it shattered many of the prejudices and mindsets that kept Alera stable but stagnant for those that had power and miserably unjust for those that didn't, and the series ends on a decidedly hopeful note.
The Battle of the Azure Nebula in the Star Trek Novel Verse, the most one-sided battle in the franchise's history.
The Dominion attacking and destroying New Bajor, and shortly after the Galaxy-class USS Odyssey. The writers invoked this trope to emphasize the Dominion threat - A starship the same class as the Enterprise stood no chance against this foe.
The Romulan entry in the war. What happened is not shown, but before that the Dominion was winning the war and on the verge of invading Vulcan, and a couple months later every single Federation and Klingon system conquered by the Dominion was back in Federation and Klingon hands (many of them overran by the Romulans and turned over to their legitimate owners as soon as they could spare some ships), and the combined Federation-Klingon-Romulan fleet was threatening the Cardassian home territory.
The Breen attack on San Francisco. Not as damaging as some of the others on this list, but shocking in that they were able to stab at the heart of The Federation.
One was planned by the Dominion as the final act of the war. After defeating the Federation they planned to wipe out all life on Earth, believing the demoralizing effect would severely limit revolutionary activities.
In some Star Trek Expanded Universe books, this happened in the Mirror Universe after the Klingon-Cardassian Alliance defeated the Terran Empire. As punishment, and an example, they turned Earth into a lifeless rock, possibly Vulcan as well.
The Battle of Serenity Valley in Firefly, which was apparently the battle that lost the war for the Independents and sent Malcolm Reynolds over the Despair Event Horizon.
In Doctor Who, much of the Last Great Time War between the Time Lords and the Daleks has only been explained through dialogue by the Doctor himself. However, we do have a definite shocking defeat that was said on the show: the Fall of the Cruciform, which was so shocking that it made the Master himself flee to the end of the universe.
There was another one. The Doctor himself mentions that he was at the Fall of Arcadia, and that he might be able to come to terms with it someday. In "The Day of the Doctor" it's revealed that Arcadia was the second biggest city on Galifrey, and its fall to the Daleks meant that the Time Lords were on the verge of annihilation. This ends up motivating the Doctor to destroy both races.
In the finale of Power Rangers Turbo, the planet Eltar falls, stripping the Rangers of their powers, forcing them into space to try and rescue their old mentor Zordon.
In World of Warcraft there are numerous examples of this, as well as a couple of subversions, but given the name of the game, that's hardly surprising. There is the War Of The Ancients which led to the destruction of the Kaldorei kingdom and the world splitting apart. Then the Orcs almost complete annihilation of the Draenei on Draenor. After which, said orcs go on to invade the world of Azeroth and sack the human city of Stormwind. Later, they go on to enslave the Red Dragon Flight, with which, they almost reduce Quel'Thalas to burning ash. Later, Prince/Death Knight Arthas ends up killing every man, woman, and child in Stratholme. After that, he kills his father, several paladins, including another father figure, Uther, then sacks Quel'Thalas and Silvermoon City to use the Sunwell, which he then blows up. Oh yeah, at the end of that campaign, Archimonde comes around to completely demolish Dalaran. The Kaldorei lose their beloved demigod, Cenarius, in a battle with Grom Hellscream. Finally, the Kaldorei give up their immortality by blowing up the World Tree to kill Archimonde. Did I mention there were a lot of examples in this?
HOWEVER the killing of Archimonde, the whatever happened to Sargeras and Azeroth's ability to merely RESIST the Burning Legion are all pretty big. And implied to be bigger than anyone knows. Given this entails two cosmic horror like beings killed in a setting every (sane) person knows to be a cosmic horror story its kinda big.
Wrath of the Lich King introduced the battle of Wrathgate, where a united Horde and Alliance force was destroyed by treasonous members of the Forsaken. This set back the offensive aginast the Lich King by years and renewed the waning war between the two factions.
Mists of Pandaria begins with Garrosh Hellscream destroying Theramore, home to the peace-seeking Jaina Proudmoore. The leader of the neutral Kirin Tor is killed and Jaina nearly destroys Orgrimmar in return. Following these events, Jaina assumes leadership of the Kirin Tor with a decidedly anti-Garrosh mindset.
By this point, almost everyone is pretty much against Garrosh, who has decided that the Horde should be an orc-only force, kicking out all of his allies. A combined force of the Horde (minus Garrosh's force) and Alliance prepares to storm Orgrimmar to take care of Garrosh once and for all. Not a legacy his father would've approved of.
All throughout the game´s expansions, the Trolls are much of a subject of this: Confirmed to be one of the first races to even be in the World. They had vast Empires, one completely shattered into two tiny tribes by Night Elves, and another almost annihilated by the first Human/High Elf Alliance, yet another, later destroyed by The Scourge in Northrend. They try many, many, many desperately evil things to try and get by, but so far, they do not seem to be able to get a break unless they align themselves with The Horde. And even that does not work THAT well for them...!
The Fall Of Reach in the Halo universe. Which is the equivalent of America's CENTCOM, Norfolk Naval Shipyard, Fort Bragg, and Norad being taken out all at once. And that's not counting the civilian casualties.
The Covenant get one of their own by losing a whole armada of about five hundred ships thanks to Bad Ass Admiral Whitcomb.
90% of the Fleet of Particular Justice was lost during the Fall of Reach. The remainder was annihilated during the destruction of Installation 04. The loss was so spectacular the Prophets laid the blame on Fleetmaster Thel'Vadamee and declared him Arbiter. One of the few times BOTH sides in the conflict had a chain of shocking defeats... at the same battles.
When Flood overrun the Covenant capital High Charity it signals a massive loss for the Loyalists and the gradual turning of the tides.
An entire city on Earth is wiped out in Halo 4 when the Didact uses a weapon that turns any sentient being into a mindless Energy Being soldier. It's only thanks to the Master Chief that he is stopped from doing that to all of humanity.
Deus Ex: the (French!) terrorist group blows up the Statue of Liberty!
Mass Effect has Shanxi, a human colony that was blockaded and besieged by the turians during the First Contact War until it was forced to surrender. The Bring Down the Sky DLC has Shepard narrowly avert one, preventing a devastating Colony Drop by batarian extremists on Terra Nova.
The Reapers plan for countless Cycles involved coming through the hidden relay inside the Citadel to wipe out galactic leadership and shut down the Relay network, allowing them to easily wipe out all galactic life system by system. Only the final act of defiance from surviving prothean scientists on Ilos prevented this from happening in the current Cycle, having altered the control signal that told the Keepers to open the Citadel relay.
In addition to the Reaper invasions across the galaxy, the fall of Thessia in Mass Effect 3.
EVE Online has the battle of Vak-Atioth, a Curb-Stomp Battle between the Jove and Amarr Empires, which sent the latter reeling into Vestigial Empire status. Later on, the Amarr reversed their fortune in the Battle of Mekhios, where they wiped out an entire Minmatar fleet and send the remnants of their forces packing.
In Dragon Age II, after the fall of Lothering to the Darkspawn horde, Hawke's family became one of thousands of refugees that fled Ferelden to escape the Blight, eventually settling in Kirkwall in the Free Marches. Part of the backstory of a Warrior/Rogue Hawke, their brother Carver and Aveline Vallen is that they were all survivors of the King's Army at Ostagar and only narrowly escaped with their lives after Loghain's betrayal.
Not the best example, since the siege failed but didn't even slow the Fire Nation's progress. More appropriate examples would be the conquest of Omashu and Azula's coup in Ba Sing Se during the second season: the latter was particularly significant as Ba Sing Se was the last free city in the Earth Kingdom.
However, considering the effect it had on the line of succession in Fire Nation's monarchy, it was an important defeat nonetheless. The city of Ba Sing Se was sieged relentless off and on for a hundred years, but Iroh's siege is the only notable attempt to capture Ba Sing Se due to it being a turning point in the history of the Fire Nation, and as a result, the 100 Year War. Iroh's loss in favor gave Ozai an opportunity to make a claim on the throne, which was successful, and as a result of Ozai's leadership, the Fire Nation came dangerously close to winning once and for all. Shocking Defeat Legacy is not a killing blow or major surprise attack, but instead a defeat that no one saw coming and one that has major consequences. Nobody imagined Iroh's son would be killed, nor that as a result Iroh would retreat. If it were any other soldier, Iroh would probably have pressed the attack and won. Or, at least, that's what everyone was expecting to happen. He might still have lost, but that's irrelevant. It's the perception of defeat at the hour of victory or safety that defines this trope.
Iroh was a much better leader than Card-Carrying Villain Ozai, and if he hadn't retired, he would've found a way to crack Ba Sing Se open anyway, either with his original siege or later (he later managed it with a much smaller army, and against an army of comet-empowered Firebenders no less). Demoralizing Iroh and pushing him away from the war and the conquering mindset was the all-important coup which led to the Fire Nation's eventual defeat. Moreover, a hypothetical Fire Lord Iroh would have encouraged massive numbers of Les Collaborateurs due to his competence, winning personality and kindness, ensuring Fire Nation dominion over the Avatar world during his lifetime at least. Instead, Ozai succeeded Azulon, but he only really cared for himself and increasing his personal power, using and abusing even his favoured daugher and his country as a whole as tools to this end and running both ragged in the process; further, with his evil antics, he put all his enemies into a very motivating "do or die" situation, as well as adding his own son and brother, considerable powers in the world especially the latter, to their ranks. Any fight against Fire Lord Iroh would have been half-hearted at best, by comparison. TL, DR: It was this trope because it replaced Iroh's solid leadership with Ozai and his Villain Ball, a major strategic defeat for the Fire Nation.
Another example was the Day of Black Sun. The Water Tribes and Earth Kingdoms had united to hit the Fire Nation at exactly the time that firebenders would be unable to bend. Everything was planned out and executed perfectly, but due to some clever stalling tactics by Azula and Ozai, the Fire Nation held out and the good guys were forced to break and retreat, and the Fire Nation could prepare for Sozin's Comet unopposed.
The loss of the Homeworlds for the Terrans in Exo Squad, although the utter destruction of Mars late in the second season was an even more devastating blow to the Neosapiens. Phaeton built most of his anti-Terran propaganda upon it afterwards.
Same thing would happen 800 years later in year 410, when the Visigoths under Alaric I sacked Rome. The Roman Empire remained as an independent nation and would live to 476, but it was clear to everyone after the sack that Rome was at that point just a shadow of its former glory self, and the only reason why the Huns were not able to finish the job was because of Flavius Aetius (who historians calls "the last true Roman")' tactical genius and his alliance with the Visigoths.
Also for the Romans, the Battle of Teutoburg Forest on September 9, 9 AD. Three Roman legions were returning to camp when they were attacked by Arminius, a Germanic chieftain who had grown up in Rome. All three legions were wiped out, and eventually the Roman Empire withdrew from Germania. Emperor Augustus, when told of the disaster, reportedly banged his head against the wall, shouting "Varus! Give me back my legions!"
More recent research, e. g. the finding of the remains of a post 9 A. D. battlefield in the middle of Germania where the Romans conclusively defeated their Germanic foes, indicates that Rome continued to make successful incursions into Germania much longer than was hitherto believed and that the eventual withdrawal from there was not immediately preconditioned on the battle of the Teutoburg Forest.
Another great Roman disaster was the Battle of Manzikert in 1071, when the Seljuk Turks routed the much larger Byzantine army and captured Emperor Romanos IV Diogenes. While modern scholars don't consider it a "turning point" anymore, the disaster at Manzikert led to the loss of most of Anatolia (some parts of which irrevocably) and plunging the Empire into a series of civil wars. Until the end of the Empire in 1453 Manzikert was widely known as "that day" and considered one of the most shameful days of the Roman Empire.
Amazingly averted by the Roman Republic in the First and Second Punic Wars. In the first, the Romans lost two entire fleets in heavy storms, losing 280 ships and 100,000 men in 255 B.C. and a slightly smaller number in 253, yet they kept building new fleets and wrested naval supremacy from Carthage. In the Second Punic War, Hannibal and his army inflicted three terrible defeats on the Romans, but Rome continued to wage war until ultimate victory. When Hannibal sent the news of his greatest victory at Cannae (216 B.C.) to the Carthaginian senate, the senator Hanno asked: "Did the Romans ask for peace? Did one of the cities of Latium or one of Rome's colonies rise against the Romans?" When Hannibal's emissaries answered "no" to both questions, Hanno said: "Then the course of the war has not changed."
Another real life example would be the Battle of Hastings, as England would've ended up an entirely different country without it. Yet another real life example (and an often fictionalised one) would be the Battle of Chi Bi in the Romance of the Three Kingdoms, as it's widely believed that if the aggressors hadn't been defeated, the Three Kingdoms period might have ended then and there, which would have had a drastic impact on China's history.
Another example, although lesser known to Westerners, but with nearly as much impact on China's history is the Battle of Fei River. Had Jin lost, the Han Chinese could have lost control of China, however, Xie An (who was famous as a great administrator and had little military experience) decided he wasn't going to let Former Qin run wild and beat back a far more experienced army well over twice the size of his own. It was the single largest catalyst in the fall of Former Qin and just ten years later practically ceased to exist.
Hastings is a perfectly justified example, in that the King of England got an arrow to the head and his troops kept falling for fake retreats. Since few people really cared who was king at the time, William the Bastard Conqueror pretty much won by default.
The Battle of Hattin 1187 to the Kingdom of Jerusalem, which wiped out almost the whole army of the kingdom. It led to the downfall of Jerusalem itself. Never again were the Franks on offensive at Outremer anymore after the defeat at Hattin.
The Mongols have done this a lot to others: they conquered China, decimated Persia, ravaged Russia, and nearly conquered Europe. But they also get the receiving end of this like in Vietnam.
Though the Turks didn't conquer Constantinople itself until 1453, a Turkish victory at Manzikert in 1071 was a significant blow to the Byzantine Empire, which continually lost territory in Anatolia to the Turks after that. This allowed the first few sieges on Constantinople, as well as the establishment of Turkish cities and military forces. By 1453, the "empire" barely stretched past the city itself (although the plague and Venetian Crusaders did damage to the city as well.)
The Loire Campaign in the Hundred Years War. Before it, everyone knew that English conquest of France was just a question of time, and the imminent conquest of Orléans would speed up things. Then the French, their courage restored by Joan of Arc, lifted the siege, launched the campaign and inflicted the English a series of defeats, the final of which, the Battle of Patay, being a Curb-Stomp Battle that crippled the English army for the rest of the war.
The Battle of Lepanto and the second Siege of Vienna for the Ottoman Empire.
The Battle of Quiberon during the Seven Years War effectively bankrupted the French government by causing a credit crunch (because financiers realized that the British could strike French trade at will). They still hadn't paid the debts off by the time of the Revolution, nearly fifty years later.
Waterloo - So famous it's practically a synonym for defeat.
Although as with many battles, what gave Waterloo such a "knock-out blow" mystique was not the battle itself but the subsequent pursuit. Thus in 1815, unlike 1812/13 and 1813/14, Napoleon's army was not given time to regroup and replenish.
Subverted with the Battle of Bailén - the Spanish destroyed three French divisions. Unfortunately, it was so shocking...that Napoleon turned up to sort the mess out in person, and promptly sent Spain into retreat.
Double subverted, because although Napoleon sent the Spanish into retreat, it was the first clear defeat of a major French army since he came to power, which (along with the defense of Saragossa) encouraged his enemies to continue their fight. Thus not only did the French army find itself stuck in a bloody war on the Iberian Peninsula for the next six years, but Austria started another war in the following year (1809), leading to the first defeat of an Army led by Napoleon in person at Aspern.
The Battle of Trafalgar became this for the Combined Navies in the Napoleonic Wars - the French and Spanish lost almost seven times as men as the British, and the majority of their active ships-of-the-line. Not only would they never challenge the British at sea again, it gave the British the ability to strike at will at French and Spanish trade, contributing directly to the collapse of both empires by slowly throttling their treasuries. Indeed, when Napoleon was presented with an embroidery of an Eagle strangling a Lion, he said that it should be the other way round. During the battle itself, the explosion of the Achille was what signaled the end of the engagement and convinced the Franco-Spanish fleet to run.
The Battle of Gettysburg was a major turning point of the American Civil War.
Although the war did continue for nearly two years after that. Many however actually see the Battle of Antietam as the real turning point of the war, because this was the last real chance the Confederacy had of not just temporarily carrying the war north (there was also a Southern offensive in the west at the time, while even if Gettysburg had been won by Lee, Vicksburg still would have fallen to Grant), but also to gain recognition from the major European powers.
Even after Gettysburg, the South still had one last chance to win, or at the very least, get a negotiated peace that would leave the Confederacy intact. That would be by keeping Union forces stalemated in sieges until November 1864, when the Northern public, sick of a long, bloody war with no end in sight, would vote Lincoln out of office and elect George McClellan President, who was campaigning on a Peace platform. In the east, Grant's forces were held at bay at Petersburg while in the west, Sherman's forces were tied up in the siege of Atlanta. But Atlanta fell to Sherman's forces in early September, boosting Northern morale and resulting in Lincoln getting re-elected. When Atlanta fell, everyone knew the South had truly lost.
The Battle of the Alamo, which was a major defeat for the Republic of Texas.
Subverted and completely reversed in that the Alamo actually fulfilled its objectives. While a tactical defeat, it was a resounding strategic victory that not only severely bled out the Mexican Army and allowed the Texan Army to organize into a fighting force. Also provided substantial morale boost.
The men of the Alamo had actually been ordered by Sam Houston to leave—Bowie's men originally came to destroy the fort, but the defenders chose to stay. Perhaps a better example from that war is the Battle of San Jacinto, where the relatively ragtag and much smaller Texan army hid out in the swamplands near what is now Houston, and defeated Santa Anna's men in a completely unexpected attack.
Early 20th Century
The disasters that were the Battles of Mukden and Tsushima were largely responsible for the Tsarist government to fold the Russo-Japanese War, despite the still enormous strategic advantage and intelligence reporting of the impending collapse of the Japanese economy. The sense of national shame still felt forty years later was one of the major reasons the Soviet Union agreed to join the war against Japan in the closing days of WWII, despite technically still being neutralnote The details of the non-aggression pact between the Soviet Union and Empire of Japan said that it would lose power only a year after its denonsation or non-prolongation, which happened in the Spring of '45.
World War I
Gallipolli for Australia and New Zealand. It's viewed in much the vein as Dunkirk for the UK, only more so.
During the Russian Civil War, Kolchak's defeat at Tobol was the turning point for the Whites to start losing. After that defeat, Kolchak's Eastern White army started a retreat that quickly escalated to panicked flight resulting in the fall of Omsk, the rebellion at Irkutsk and Admiral Kolchak's own demise. For Denikin's Southern White army, the failure of his Moscow offensive at the battle of Orel was such a defeat; after Tobol and Orel the Reds had a practically guaranteed win, and the rest of the war was basically cleanup of remnant Whites, Blacks and Greens.
The sinking of the SMS Szent István during World War I was this for the Austro-Hungarian Navy. Before that, the Austro-Hungarians had tried various times to break the Otranto Barrage. Then the Szent István, the flagship of the fleet, was sunk by two Italian torpedo boats that just happened to be in the area, and not only the attack the flagship was supposed to lead was canceled, but the Austro-Hungarian navy didn't dare to leave the ports anymore.
Not that there was much time left for another attempt - the Szent István was sunk on 10 June 1918, less than five months before the land battle of Vittorio Veneto which forced Austria-Hungary to sue for peace.
Subverted by the Battle of Caporetto of World War One. While 'Caporetto' is still synonymous with 'complete and utter defeat' in Italian and the Italian Army was forced to cede half of Veneto to the Austro-Hungarian invasion, the Italian soldiers, upon noticing the civilians were running from the invaders, rallied up at the Piave river and stopped any attempt to pass it for a year. One year later, the Battle of Vittorio Veneto was one for the Austro-Hungarians: while the Austro-Hungarian Army quickly recovered from being dissolved (as pretty much all fighting units had been disperded and routed, but managed to reassemble fairly fast), the Austro-Hungarian Empire did not, and collapsed three days before the end of the battle (and two after begging for an armistice). Furthermore, the terms of the armistice (that included free passage to Germany through Austria) forced the German Empire to ask for armistice, as the plan to fight through the winter to get favourable peace conditions had been shot to hell by the threat of a million battle-hardened soldiers from the South.
World War II
The Battle of Midway became this for the Japanese, as their loss in the battle effectively halted their expansion and put them fully on the defensive for the first time. US Naval Supremacy was only a matter of time, however; the Empire couldn't stay lucky forever in the face of the Allies' overwhelming economic and industrial superiority.
In a sense, the Doolittle Raid counts as well. After Pearl, the Japanese appeared invincible, seizing island after island and colony after colony. By April 1942, they had all but swept the Allies from the Pacific. Then, sixteen B-25s were transported by carrier a few hundred miles from Japan and dropped light bombs on Japanese cities. Almost no infrastructure damage was inflicted, but it scared the crap out of the Japanese. This drove Admiral Yamamoto to fight much more aggressively, causing him to attack Midway in order to secure it and thus the Japanese defense perimeter would be complete. Then, at Midway, all four of Yamamoto's carriers were sunk...
The raid also had a vital strategic effect, albeit it wasn't appreciated or understood at the time. Japan's home defense fleet was very weak, so they recalled a fleet that was heading for the Indian Ocean. This gave the Royal Navy a much-needed breather to regroup and regain its strength in the Pacific.
Britain's 1940 retreat from Dunkirk marked a turning point in World War II, but is remembered in Britain more for the heroic rescue of stranded troops than as a defeat.
The "Fall of France" fits the trope name better. Dunkirk is more of snatching victory from the jaws of defeat (although Churchill was quick to point out that you don't win wars with retreats and that while it was great to save the troops, the Fall of France was still a major defeat for the Allies).
Also, they had to leave most of their heavy arms, fuel, ammunition, vehicles, and equipment behind (which was enough to supply eight to ten divisions), so it would take a while before they were in a condition to face the Wehrmacht in the field again.
The failed Rzhev offensive and loss of Sevastopol of 1942 were probably this for the Soviet Union. They were later overshadowed by the great victories of Stalingrad and Kursk, but these failures, which stemmed largely from overconfidence after the successful Battle of Moscow and generally crappy Soviet logistics, costed enormously both in lives and materiel, and later weighed heavily over Soviet morals and military thinking even late in the war and after it.
For the European colonial powers in general, and the UK in particular, the Fall of Singapore. It had been boasted that Singapore was the best-defended city in the world, the armour-piercing shells of her heavy gun emplacements capable of punching through any ship the Japanese had to offer. However, Singapore was besieged from the landward side, and their anti-battleship defenses were wholly ineffective at targeting infantry. Percival had 100,000 troops on paper, but they were in no shape to continue fighting. Realising that their situation was hopeless, he surrendered. Only to find that the Japanese only numbered some 30,000 and their supply situation had been even worse. It was this event (among others in Southeast Asia) that inspired the acceleration of the decolonisation process - the European colonists were not, and never had been invincible.
Churchill himself felt the fall of Singapore to the Japanese in 1942 was the greatest defeat in British history.
Winston Churchill:"I put the telephone down. I was thankful to be alone. In all the war I never received a more direct shock."
It's generally acknowledged that even had Percival pushed the issue, he felt that Pyrrhic Victory would not have been worth it.
The Battle of Stalingrad, for the German forces in the Soviet Union. So much so that this was the first time the Nazis publicly acknowledged that the war was beginning to go badly.
Despite the loss and withdrawal from El Alamein, the campaign in North Africa continued for another six months. When Rommel ordered the retreat, Montgomery did not pursue and thus allowed the broken German formations to withdraw, fortifying themselves within Tunisia. The loss of Tunisia occurred only a few months after the loss at Stalingrad, with a quarter of a million troops taken prisoner. Soon after, the loss of Sicily was so great that the Italians overthrew Mussolini.
Rommel probably thought he could regroup and attack again the following year after halting the British offensive. What made El Alamein ensure this would never happen was that not too long after, the Americans started landing in Morocco and Algeria.
After failing to break through to Alexandria and beyond in the First battle of El Alamein, Rommel would have had to retreat after the American landing in Morocco and Algeria in any case. Some people therefore see the Second Battle of El Alamein as having been fought to a large extent to build up British morale and prestige in a last chance to win a major battle without the American army helping...
The Night of Taranto during World War II. Before it, the mere threat of the Italian battlefleet stationed at Taranto was making the Royal Navy cower. After a carrier attack neutralized half of the Italian battleships (including the newly completed Littorio, the most powerful battleship in Europe, damaged and neutralized for five months), the balance of power was firmly into British hands, and the Italians never managed to win a decisive engagement.
Incredibly and unbelievably Subverted in the Italian revenge for that, the Raid on Alexandria, in which six frogmen with three manned torpedoes penetrated Alexandria's harbour and sank two battleships and damaged a tanker (plus a destroyer by accident: it was too near the tanker when the mine placed on it by the frogmen exploded). In theory the Mediterranean Fleet had been neutralized, and the Italians were free to dominate the Mediterranean Sea until the battleships could be repaired or replaced... Except the Italian high command failed to find out: the frogmen had been all captured before they could report their success, and the British immediately raised the battleships (sank in shallow waters) and patched them up enough that air recon believed they had not been damaged. While the raid did mark the start of six months of Axis victories in the Mediterranean, had the Italians knew of it nobody knows what they would have done.
Vietnam was the greatest military quagmire in United States history.
Within the war itself, the Tet Offensive can be considered this. The Vietcong attacked many cities simultaneously, notably claiming the US Embassy in Saigon. This was a major hit to the vision of American strength and hit their morale deeply, especially as nightly TV news had shown repeated US victories over the Vietcong militarily speaking. In reality, the Vietcong suffered heavier losses than the American troops and the attack itself devastated the Vietcong's ability to operate to such an extent the North Vietnamese Army took over operations in South Vietnam, but in terms of perception this drastically swayed the war in their favour.
The 2004 Dream Team during the 2004 Olympics, who were soundly beaten by... Puerto Rico. And then, worse yet, Lithuania. The media refused to let American basketball players forget, until they got it together for the Redeem Team run in 2008.
The New England Patriots are best remembered for their 18-1 season. They were undefeated heading into the Super Bowl, and they lost the game to the underdog New York Giants on a flukey catch.
The Miami Heat, led by Le Bron James, Dwyane Wade and Chris Bosh, ran roughshod through the Eastern Conference in the 2011 playoffs, defeating even the Boston Celtics and the Chicago Bulls in five games apiece. Entering the Finals against Dallas, the Heat led the series 2-1... before losing the next three games to the Mavericks. It especially stings when you consider LeBron saying that he'd win multiple championships with Wade and Bosh. The media, the fans and the internet never let the Heat live it down after that, and the loss was haunting enough that it sent LeBron past the Despair Event Horizon. (he lead the Heat to the title the following year winning basically every award in the process, but the legacy remains because he left Cleveland solely to finally get a title, only to fail again with a better team)
It didn't help that the Heat seemed on the cusp of seizing victory for the series in both Game 2 (which would have severely crippled Dallas' morale) and Game 4, only for Lebron to get a little presumptuous by celebrating with Wade next to the Dallas bench in Game 2, leading to the miracle comeback that was led by Dallas superstar Dirk Nowitzki. From there, the Finals were a hard-fought battle to the finish.
Speaking of Cleveland, their teams have plenty of this, helped by the fact that most of their notable defeats can be summed up with a single phrase (The Shot, The Fumble, The Drive, Red Right 88, The Slip). Two were even off-field (The Move, where the Browns were moved overnight to Baltimore; and The Decision, where LeBron announced his departure).
The World Cup has at least three finals, 1950 (Brazil loses at home; 5 titles later, it's still a sore point), 1954 (Dark Horse Victory of Germany over the heavily-favored Hungary) and 1974 (Dark Horse Victory of Germany over the heavily-favored Netherlands... though not as unexpected as the previous one).
Previously, both the previous World Cup's champion and the host country of the upcoming World Cup were given automatic spots in the upcoming World Cup. Then France, winners of the 1998 World Cup, turned in an atrocious performance in 2002, earning only one draw in three group games and failing to score a single goal. Since 2006, only the host country gets a free pass.
In 2014, Brazil is the host nation, their first home tournament since their 1950 defeat (itself already a Shocking Defeat Legacy). With their new superstar in Neymar, Brazil was the heavy favorites for the World Cup - although it was nip and tuck a lot of the way. They made it to the Finals, but they had lost Neymar to injury, forcing him to miss the rest of the tournament. As a result, Brazil gets utterly humiliated by Germany with a score of 7-1 in front of their own native fans, which is both Germany's greatest margin of victory and Brazil's greatest margin of defeat, as well as one of the most lopsided games in World Cup history. Brazil would have to settle for a third place finish against the Dutch... only to lose yet again 0-2. This will not ever leave Brazil's consciousness for quite some time.
For most of The Eighties and the first half of The Nineties the New York Yankees were in a Dork Age (relatively speaking for a franchise now with 27 World Series championships), having not made the postseason between 1982 and 1993note they most likely would have made it in 1994 with a 6 1/2 game lead in the AL East had the players' strike not cancelled the last few weeks of the season. In the '95 season they made it as the AL Wild Card and seized a 2-0 lead in the best-of-5 series against the Seattle Mariners before dropping Games 3, 4, and 5 at the Kingdome, the latter on a 2-run double in the bottom of the 11th inning. With a tempermental owner in George Steinbrenner, the Yankees brought about large changes to the starting roster both from within (i.e., Derek Jeter and Mariano Rivera) and without (i.e., trading for, among others, Tino Martinez, who was a member of those '95 Mariners), as well as firing manager Buck Showalter and replacing him with Joe Torre. With a new core in place, the Yankees would win four of the next five World Series.
Bill Simmons wrote a couple of articles about the Levels ofLosing (first article from 2002, second one from 2007 following the New York Mets' September collapse in the NL East), with Level 1 being reserved as the sole domain of "That Game" (Game 6 of the 1986 World Series). Then the Brett Favre-led Minnesota Vikings choked away the 2010 NFC Championship Game to the New Orleans Saints (the batch of emails in the article conveys quite nicely the devastation on Vikings fans' psyche), and Simmons went back and made a list of tortured teams eligible for a loss of that magnitude.note Please note that since then the Seattle Seahawks and San Francisco Giants have won championships, and so would no longer be eligible by his criteria.