Robert Oppenheimer

The bombs were dropped on Japan on August 6 and 9, 1945.

An excerpt from the article 32 facts about Robert Oppenheimer

President Harry Truman authorized the dropping of "Little Boy" on Hiroshima on August 6, 1945, killing approximately 66,000 people. "Fat Man" was dropped on Nagasaki on August 9, 1945, killing 42,000 people. 

On August 14, 1945, Emperor Hirohito of Japan signed an unconditional surrender.