Hi there!
This depends on what time period and war you are talking about.
World War I - The first bomb dropped on May 31, 1915 by a German zeppelin on North London.
World War II - The atom bomb was dropped on Hiroshima and Nagasaki on August 6, 1945. This was a game end that stopped the war in the Pacific.
Cold War - No bombs were actually dropped, but both the U.S. and the Soviet Union had weapons to back up their threats. Truly, it was a game of poker and everyone was bluffing.
Hope this helps!
- Chris