Ruins of Hiroshima

On August 6, 1945, Hiroshima became the first city in history to be destroyed by an atomic bomb.

Even with hindsight it’s impossible to be sure whether the use of the bombs over Hiroshima and Nagasaki led to the post-war nuclear arms race or whether the bombs demonstrated their horror in a way that no harmless experiment could ever have done, and so helped to hold the ultimate violence in check throughout the Cold War.