In 1945, the atomic bomb was successfully developed and two were dropped on Japan, causing millions of deaths and bringing World War II to an end.
From that moment on, Einstein faced the reality of world-ending, human-destroying superweapons and fell into a deep depression and regret. He profoundly regretted the letter he wrote to Roosevelt, which had opened Pandora's box for humanity and granted them the terrifying power to destroy themselves.
In 1952, humanity developed its first hydrogen bomb, a weapon hundreds of times more powerful than the terrifying atomic bomb. The Little Boy atomic bomb, with only a 20 kiloton yield, had destroyed an entire city. What, then, could a hydrogen bomb with a yield of 10 megatons destroy if used in war? Einstein realized Pandora's box, once opened, could not be closed, and he became increasingly melancholic.
