The use of atomic weapons.
"“That single bomb, weighing 8,900 pounds, wiped out nearly five square miles of Hiroshima–60 percent of the city. More than 78,000 of the city’s total population of 348,000 were killed; an estimated 51,000 were injured or missing.” " “The Japanese announced their acceptance of unconditional surrender on August 14. World War II officially ended at 10:30 a.m. Tokyo time, September 2, 1945, when Japanese emissaries signed the surrender document aboard the battleship USS Missouri in Tokyo Bay.” " And ". “Atomic bombs have been used only twice in war—both times by the United States against Japan at the end of World War II.”
Hope this helps
If you mean what LEAD TO the end of the war, I would say that would be the fact that Germany lacked the resources to keep going, because, after 1941, they were fighting the US, the UK, Italy, Russia and a few other countries mostly by themselves.
A big turning point in the war was the German defeat against the Russians in the battles of Kursk and Stalingrad. The Nazis were in gradual retreat after that.
One thing about Japan, yes the atomic bombs ended the war, but Russia also invaded at the same time. You might not think that would matter, but I've read about it, and apparently the Japanese had fought a bit against the Soviets in 1939 before WWII started and the Japanese lost very badly, so the Soviets were a factor in the Japanese surrender too.
I am a history major, so this is all from memory, but it should be easy to check by searching some of the key names.
