Answer:
First and formost was the invention of Nuclear Bombs. This 1 bomb came about to end it. Now... The united states alone has enough to end all life on earth. But 80,000 people alone died in hiroshima in 1 bomb attack. Also not to mention the use of napalm on Japanese cities. Also the bombing campaigns didn't target JUST military it also killed civilians. Also have to look at the v1 and v2 rockets. The precursors to ICBM missiles and the "Killing Chambers" of the concentration camps. This war was killing on a very large scale. 50 million people so it's said ... I believe it to be more.