Discussed in the opening narration of A Bridge Too Far: 'The plan, like so many plans in so many wars before it, was meant to end the war by Christmas and bring the boys back home.' See below for Operation Market Garden, the Real Life example on which this film was based.
In Stalingrad, one of the German soldiers says they'll take the city in three days. In three weeks, the city still won't have fallen. In three months, most of the soldiers will be dead.
In the Discworld book Jingo, the Upper Class Twits leading the Morporkian army thought that they'd have a jolly good time teaching the Klatchians a lesson, that the enemy would run at the sight of their steel, and that they'd be home by Hogswatch. This despite the fact that they were badly armed, hopelessly outnumbered, and the Klatchians who actually saw their steel tended to start sniggering.
Later in the book, one character bemoans that "We were going to sail into Klatch and be in Al-Khali by teatime, drinking sherbet with pliant young women in the Rhoxi," mirroring the confident statements of some WWII soldiers that "We'll be drinking coffee in Berlin by teatime."
In The Short Victorious War, The People's Republic of Haven leaders decide to resort to war against Manticore. A short, victorious war, they believe, will both distract the proles from their current economic problems and allow them to use the riches of the Manticore system to prop up their welfare state. Needless to say, the war is a disaster for Haven and ends up lasting for almost 20 years.
They even lampshade this with a quote from V.K. Plehve (see the Real Life section below), providing the Title Drop for the book in the opening epigraph, followed by the page quote by Robert Wilson Lynd.
In the George Orwell novel Homage to Catalonia, a Spanish government general predicted that he and his men would be having "coffee in Huesca" just a day following the taking of Sietamo during the Spanish Civil War. As for how the battle in question went, well...
Months earlier, when Siétamo was taken, the general commanding the Government troops had said gaily: 'Tomorrow we'll have coffee in Huesca.' It turned out that he was mistaken. There had been bloody attacks, but the town did not fall, and 'Tomorrow we'll have coffee in Huesca' became a standing joke throughout the army. If I ever go back to Spain I shall make a point of having a cup of coffee in Huesca.
The BattleTech universe certainly has seen its share of this trope. Two large-scale examples — smaller ones abound, but these are really just perhaps the two biggest ones in the entire history of the setting — would be the Reunification War (as initially viewed by the newly-minted Star League) and the Clan invasion of the Inner Sphere (from the original point of view of the Clans); needless to say, neither actually worked out that way.
The final episode of Blackadder Goes Forth (the fourth and last Blackadder series, set during World War One) features characters making remarks like 'It's ice cream in Berlin in 15 days', 'See you all in Berlin for coffee and cakes', and 'We'll be sucking sausages in Berlin by teatime!' about the upcoming battle. Eventually Blackadder hangs a lampshade on this by snarking, 'I hope their cafés are well stocked; everyone seems determined to eat out the moment they arrive.'
Studio 60 on the Sunset Strip: during a two parter where a cast member's brother is being held hostage in Afghanistan, we see a Flash Back, during which the President of the network (who was only a VP then) opines that the war will be over in a month.
The Battle of Serenity Valley in Firefly was assumed by Alliance command to be an easy victory. Though the Independents would ultimately lose the battle due to massive Alliance air support, the browncoats were nonetheless able to hold the Alliance for far longer than anyone thought possible (the fact that the Alliance had to call in the air support was a victory of sorts). Unfortunately, it's implied that the "victory" in the valley was ultimately disastrous for the Independents, who surrendered not long after.
In the NCIS episode "One Shot, One Kill", a Marine recruiter tells a pair of nervous potential recruits, "What with boot camp, S-O-I, follow on schools... we're talking over a year and a half of training. Iraq will pretty much be over by the time you boys graduate." The episode aired in February of 2004, and Reality Subtext ended up putting that assertion squarely into this trope.
In the opening scenes of the German miniseries Unsere Muetter Unsere Vaeter, five friends have one last drink in Berlin. Two of them are leaving for service in Operation Barbarossa, one of them has volunteered to be a nurse on the front, another is an aspiring singer, and the last one is Jewish. They leave, hoping that by Christmas, Russia will be crushed and all of them will be reunited in that bar. The war ends four years later, Germany is defeated, and two of the friends don't make it back.
The Covenant from Halo believed that their war against humanity would be a short venture. Instead, the war would drag on for another twenty-seven years with no end in sight for either side and would divide Covenant society to the point of Civil War (the opposite effect was true of the humans, whose many factions, after years of fighting amongst one another, generally agreed to unify against the common threat of genocide by the Covenant war machine, though there was still fighting between the groups as of 2552).
As revealed by backstory fluff, the Taiidan Empire from Homeworld attacked Kharak in the belief that a short victorious war against the surviving Hiigarans, ancestral enemy of the empire, would have quelled the internal opposition by both finishing the job with their most hated enemy in history and showing the might of their all-powerful emperor. Not only the destruction of all life forms of Kharak triggered the revolution and civil war (as the people realized their ancestral Hiigaran enemies had forgotten the treaty forbidding them from using hyperspace technology ever again that they had broken), but some Kushan, the descendants of the Hiigarans, had survived, and would cause the collapse of the Taiidan Empire by killing the emperor in battle.
In the story "Heart of the Lion" from The Wulf Archives, the White Empire attempts to conquer the Veldt Lands in order to enslave the natives. This "quick campaign," led by an arrogant Upper-Class Twit by the name of Lord Heatham, fails miserably — less than half of the forces fielded actually make it to the battlefield due to adverse sea conditions and local diseases on land, and the battle with the Sholanti ends with everyone on the battlefield (except the protagonist) slaughtered to a man.
At the start of The American Civil War, both sides assumed it would end in a quick victory. Confederate Secretary of War Leroy Pope Walker even predicted he would be able to mop up all the blood spilled with a handkerchief.
In the same war, there was a battle in New Mexico Territory called the Battle of Glorieta Pass. When the two sides met, a Union soldier reportedly yelled out, "Out of our way, rebels! We'll take our dinner in Santa Fe!", to which one of the rebels replied, "You'll take it in Hell!" The Confederate soldiers actually won the battle, but were forced to withdraw back south after one of the Union leaders, Maj. Chivington, located and destroyed the rebel supply train.
In 1903, on the eve of the Russo-Japanese War, V.K. Plehve, Russian Minister of the Interior, said: 'What this country needs is a short, victorious war to stem the tide of revolution.' The war was neither short nor victorious for the Russians, and the revolution came fourteen years later.
The start of World War One is probably the most egregious example. All sides were confident that their soldiers would come home victoriously within months - as we all know, nothing of the kind happened, and the war lasted four years.
The Central Powers' reasoning was actually sound, as their ally Italy was supposed to attack France on the south, thus preventing the French from concentrating enough troops against the Germans on the north and allowing the Triple Alliance to knock their enemies out of the war one by one. Italy declared neutrality, and we know how it went.
Played straight and Subverted at the same time when Italy finally entered the war, even if on the opposing side. Politicians and patriots expected a quick and easy victory, with an almost immediate conquest of Trento and Trieste and maybe even a conquest of Vienna itself. The Alpini mountain troops (recruited on the same mountain border area where they would fight the Austro-Hungarians), on the other hand, knew they would fight for months and maybe years, and the Italian commander-in-chief Luigi Cadorna, while hoping that the Austro-Hungarian need to hold the Eastern Front against Russia would allow him to actually pull it off, had planned from the start for a prolonged war where the superior reserves of manpower of the Italian Army would gradually wear down the Austro-Hungarian forces to collapse.
Later on in World War One, the Germans tried to force a breakthrough at Verdun. Their plan was to shell the French trenches empty (of live Frenchmen, that is), then rush in before the French could bring reinforcements to the scene. German troops were unpleasantly surprised when they found that a handful of French soldiers had survived the bombardment and fought back bitterly, holding out long enough for French reinforcements to arrive. The result was a long battle which came to be seen — especially in France — as one of the most definitive examples of War Is Hell.
At the beginning of the Winter War, Stalin was expecting Finland to surrender in a couple of weeks - after all, it was a small backwoods country up against a world power - but the war went on for three and a half months and ended in a Pyrrhic Victory for the Soviets: they did force the Finns to concede their border territories, but at a huge cost in both lives and international prestige. In addition, this "victory" was the key reason Finland allied with Germany in World War II (the Finns even call it the "Continuation War").
Subverted in the early stages of World War II, in that Germany really did conquer much of Europe without much effort. Even the USSR suffered blow after blow at first; what saved them was just how large the nation is. The Wehrmacht simply couldn't reach Moscow before the winter kicked in, and Russia still had two thirds of the country to retreat to. In addition, the Russians had wisely relocated much of their industrial base further east before the beginning of the war.
Also played straight at the same moment, as Germans did not expect such a fierce, if incompetent, resistance. Without it, even the vast spans of land would do little to stop the Wehrmacht. The Blitzkrieg was essentially dead by the August, and the meat grinder settled in.
Just before Operation Barbarossa started, Hitler told his generals, "All we have to do is kick in the door, and the whole rotten structure will come crashing down." Given how poorly the Red Army did against Finland, and how quickly Germany was able to do what took Napoleon several years, the rest of the world could not disagree with that sentiment.
Played straight later on in the same war. The Allied high command were very confident about Operation Market Garden; however, they had not counted on the two German armour divisions who were resting up near Arnhem and Eindhoven, respectively. Warnings from the Dutch Resistance about the presence of these troops were disbelieved.
Operation Market Garden had much more complex reasons for being initiated, as well. Eisenhower felt that he had to launch the operation to appease his superiors (the civilian politicians who wanted the war over as quickly as possible). He felt that a more measured approach, steadily winning ground from the Germans, would have been better, but he instead committed most of his forces to the operation in an attempt to secure a route straight to Berlin. In fairness, had the operation succeeded, the war would have been over by Christmas of 1944, possibly long before. The failure of the operation and the sheer amount of resources committed to it, which then tied up more resources in an attempt to save the men who were cut off from Allied aid, meant that the war would drag on until May 1945.
Despite the many errors in both planning and execution, the sheer bravery and determination of the soldiers involved brought Operation Market Garden far closer to success than it had any business getting. But that didn't actually matter, because nothing less than total success could have made the operation worthwhile. In terms of the actual results, almost succeeding was no more useful than being utterly crushed from the start would have been.
When Boris Yeltsin's popularity declined, he thought he needed a quick military victory to boost his prestige. The Chechen rebels would do nicely... or so he believed. The result was a long and bloody war which ended in humiliating defeat for the Russian army, ultimately doing little more than showcasing that the once-feared Red Army was not even a shadow of its former self.
Things were much more complex really. The main reason for the First Chechen War was not so much a Yeltsin's wish for a prestige, as is was the attempt to avoid a further national decay. There were a lot of separatist movements in Russia at the time, and the Chechens, who were attacking civilians and engaging in their own ethnic cleansing campaign, were just most dangerous of them. The war wasn't also as much lost as it turned into a bloody quagmire The Government decided to wash its collective hands off of. See The War on Terror (well, the less favorable portrayals thereof).
Before the 1948 Arab-Israeli War, one Arab general infamously claimed, "This time next week we will be eating lunch in Tel Aviv." Turns out, not so much.
During the Second Sino-Japanese War, the Japanese predicted that Shanghai would fall in three days. However, they were not prepared for the strength of their opposition, including a million of Chiang Kai-Shek's elite soldiers trained by German advisors. It took three long and hellish months involving fierce urban warfare before Shanghai finally fell.
During the Korean War, General Douglas Mac Arthur was quoted saying this almost word-for-word about the progress of coalition forces. Then the Chinese entered on the opposite side and the war was stretched out for a few more years.
A complaint occasionally fielded about the American public during the War On Terror, especially in the early years, was that they expected this despite warnings from US military officials that they were in for a very long campaign. Blame for this is typically laid upon the surprisingly easy victory in Desert Storm, where the American-led coalition steamrollered over the Iraqi forces, and poor planning by civilian politicians, who expected to be greeted as liberators by the Iraqis. Then again, the Gulf War of 1991 was a very different sort of war from the wars the US would be fighting in the early 21st century.
Cleverly averted when the Americans left Iraq, however. The time table was "by the end of the year", even though they were gone by Christmas. Genre Savvy General, perhaps?
While no timetable was ever mentioned, American War Hawks at the beginning of the War of 1812 declared the conquest of Canada to be "a simple matter of marching." All invasion attempts ended in failure, though the Americans did succeed in burning York (now Toronto), the capital of Upper Canada.