World War II for most of the Allies. They won, obviously, but even countries that got off 'lightly' tended to suffer a casualty rate in the range of tens of thousands, the majority with over 100,000 dead, the major ones with over one million dead, Soviet Union and China with over ten million dead, a grand total of approx 50 million (no one knows for sure). Oh, and even as World War II ends and everyone is celebrating, the Iron Curtain descends and the world is plunged into an additional 40 years of the Cold War.
In the Eastern Bloc this was even more notable as they lost more people as a percentage of their populations than anywhere else. Poland and the western portion of the Soviet Union (Ukraine, Belarus, Lithuania) lost on average about 15% of their total populations. And then they were under the thumb of Stalin. The gulag was busy in the immediate post-war period.
And the bombing of Hiroshima and Nagasaki as well. While it may have ended the war, it came at the terrible and high cost of hundreds of thousands of innocent lives being lost.
World War I for the Allies as well. France lost over a million young men, mostly in battles on home soil, and became so terrified of another such conflict that the defeatism that crippled her during the Thirties and Forties set in. Britain's world empire was severely shaken, several colonial jewels gained their independence, and she went from being a creditor to a debtor nation. Italy lost 600,000 soldiers to gain trifling bits of land, fueling its resentment and hatred for the war-spoiled Western allies that would lead to the Duce. Serbia suddenly found herself going from a nonexistent nation to total control of the western Slavic peoples, and ruled them with an iron fist, creating deep-seated ethnic hatreds. Russia collapsed altogether. Greece, which participated in little actual fighting, went mad with glee at Turkey's supposed weakness at war's end, invading Anatolia and complicating political matters in the eastern Mediterranean for many years. The latecomer United States was shocked by the carnage into adopting a policy of isolation. Japan was perhaps the only Ally not to be horrifically scarred by the war, and their experiences in it led to the military takeover of the Japanese government and the road to war again.
The Winter War between the Soviet Union and Finland. Soviet Union gained ground to secure the city of Leningrad... but suffered over 126,000 dead in the effort, with hundreds of thousands more wounded, many of them maimed for life, and failed to occupy the whole of Finland. On the other hand, Finland preserved her independence... but lost 25,000 dead, and drifted in the direction of the Nazi Germany.
The Cold War and Hole in Flag revolutions. The Soviet Union broke up, dictatorships fell, and a tense nuclear situation was averted, but it left Slavic people with widespread poverty throughout the 90s and privatization left much of its successor state's wealth in the hands of oligarchs. Not to mention, ethnic tensions bubbled up to the surface and left us with 10 years of Yugoslav wars.
Some former Soviet countries, like Ukraine, have yet to recover their GDP as it was upon their independence. This is over 20 years after the fact. Moldova is the poorest country in Europe and its per capita GDP of about 1500 US dollars makes neighboring Romania seem wealthy.
Also, while the Cold War did put an end to one of the most oppressive and violent empires in history, it also left many third world nations across Asia, Latin America, and Africa scarred from the proxy wars the Soviet Union and the United States fought there.
Once war breaks out, the best possible ending is bittersweet... in a large scale. If it's imperialism and you're the empire, jolly good for you.
After any major disaster. ANY. New Orleans after Katrina, New York after September 11, 2001. Hiroshima and Nagasaki after the Bomb, the Gulf Coast after the Deepwater Horizon oil spill, etc. Yes, you may stop/survive the damage, but after large-scale disasters like that nothing goes back to the way it was.
Someday, billions of years in the future, planet Earth will be wiped out, humankind likely having died off long ago (assuming they didn't flee to another planet). But, in a few more billion years, circumstances that made Earth will repeat, creating another, new Earth, and new life will be born again. It's just a matter of waiting. And the universe will wait because, ultimately, time is the only thing it has.
And yet, something new will, always and forever, be born.
And, due to weirdness, even that won't stop the universe from continuing. (It'll take a damn long time, though; a few quintillion years, perhaps?)
Also, Hubble's Law states that the universe is expanding, and with the discoveries of several other Earths, anything is possible!
The K-Pg. On the one hand, it cleared the way for mammals, and eventually ourselves. On the other, it wiped out the non-avian dinosaurs, not to mention pterosaurs, mosasaurs, ammonites, and many others.
Earlier, the Permian extinction. It opened the way for all lifeforms that survived, but eliminated nearly all synapsids, bizarre amphibians and many, many other living beings that would never have parallel forever.
Losing your innocence could count as a bittersweet ending to your childhood.
Key word: could. It really depends on 1) what you count as "innocence" and 2) what your childhood was like in the first place.
In the context of Canadian Politics, NDP supporters, BQ critics, and perhaps even those who are neither might consider the more recent Canadian election bittersweet. Jack Layton's campaigning won over an increasing number of NDP votes in Canada, taking votes from many other parties, especially the BQ, and Jack Layton became official opposition leader... but died soon afterwards. Pushing the BQ downwards and the NDP to second-place was a huge change to Canadian politics, but the leading force for it only got to see a couple months' worth of the results.
The Battle of Trafalgar was this for Great Britain. Britain was safe from invasion, Bonaparte's Europe could be blockaded by the world's largest navy. But Horatio Nelson was dead, and a storm meant that barely anyone got paid prize money.
Basically any war or battle can be this for the winning side, especially for the relatives of those who died in it. As the Duke of Wellington put it, the only thing sadder than a battle won is a battle lost.
Pick a crash, an accident, a major disaster. Any documentary covering the incident will most likely note the safety changes made as a result, usually in the end. Still, it leads to the Fridge Horror that it seems that nobody would notice design failures unless a crash happened - more so if the changes were major and the fatalities were abundant.
One of the most wrenchingly ironic examples of this is the 1993 World Trade Center bombing. It took forever for the towers and surrounding buildings to be evacuated. As such, new safety measures and procedures were implemented. Eight-and-a-half years later, following and using these new safety measures, the vast majority of people below the impact zone of the towers were able to escape from the buildings relatively quickly and mostly unharmed.
Longtime Pittsburgh politician Bob O'Connor had dedicated most of his adult life to becoming Mayor. He came close during a bruising primary election in 2001. He came back in 2005 to win the whole thing and seemed to be off to a great start and he had high popularity. Seven months into his term, however he finds he has incurable brain cancer and he dies after only 9 months on the job that he'd tried so hard to get.
For some kids, when you suffer a lot of ups and downs in high school and you end up graduating and leaving for good.
Graduating university counts as well. You've finally gotten that degree you worked so hard for and are probably looking forward to making something of it, but you'll leave behind campus and living close to your friends. There's a reason that a lot of people regard college as the best years of their life.
Winning an auto race or championship during which another driver died, such as at the end of the 2011 Indycar season where Dan Wheldon died and Dario Franchitti, a dear friend of his, took the championship when the race was canceled.
If you die of old age, death can be interpretted as this. Yes, it's all over now. But you've lived a long life and seen many things. You haven't died young, or through anything torturous or painful. You're scared, sure and you don't know what come after death. But you've had your life and the chance to experience many things.