American, but answering anyway.
It's complicated. American arms manufacturing cements the United States as the super power until Russia developed the bomb. Stalin sacrificed many lives in the name of the Great Patriotic War, but did so with American trucks, fuel and arms.
Stalin played the allies, especially Churchill and Roosevelt, both of whom thought they were playing Stalin. In the name of realpolitik, the allies sacrificed democracy and freedom in places like Poland and Lithuania, despite the fact that the Polish Underground's gift of a German Enigma machine led to a significant advantage to the allies in the European theatre.
Was the bomb necessary against Japan? Who the hell knows. It's a damn shame it came down to it though, as atomic and then nuclear weapons are truly horrifying.
Not enough was done to stop the Holocaust. There was plenty of evidence that something horrifying was going on, but I think the Allies didn't want to believe the scale of what was being reported.
Not enough was done to stop the atrocities the Japanese would commit on the Chinese. Even Mao didn't lift much of a finger to help, so that he would be in better position after the war. To this day the West doesn't focus as much on these atrocities, which is a shame. Unfortunately I don't think anyone has learned the importance of stopping such genocidal tragedies. We still sit on our hands in the name of real politick when the major powers are in better shape than ever to punish evil regimes who would systemically murder.
Long term? Bizarrely, Japan and the US now enjoy a good relationship. As does Germany with most of it's WW2 time enemies. Unlike WWI, where an indecisive war of attrition led to "stabbed in the back" myths, none of the losers of WWII could spin a story of anything but utter defeat. The Marshall Plan and other rebuilding efforts in the destroyed countries of World War II seems to have been a stroke of genius.
Long term part 2? I'm stealing this from a video I watched on World War II, but no one has been willing to pull the trigger on a major war since WWII. There's been smaller conflicts and gnashing of teeth among the big powers, but no one wants another World War II. Hopefully that sentiment lasts.