My first cousin, twice removed (he and my grandfather shared the same namesake), made a monumentous decision in 1945. Harry Truman authorized the dropping of atomic bombs on Hiroshima and Nagasaki.
Following the horrors of island hopping towards Japan, especially on Okinawa (and the high loss of life on both sides), it was believed probably correctly that an invasion of Japan's mainland would result in unprecedented slaughter. Dropping the Atomic Bomb on Japan was seen as a way to bring the war to an end quickly, and thereby save millions of American and Japanese lives. I do not argue the decision.
It was also undoubtably seen as a way to warn the Soviet Union.
On August 6th, 1945, the Feast of the Transfiguration, the Enola Gay dropped "Little Man" on an unsuspecting Hiroshima. The shodows of people were permanently etched onto walls.