Final answer:
The U.S. dropped an atomic bomb named "Little Boy" on Hiroshima on August 6, 1945, which was authorized by President Truman and led to Japan's surrender, ending World War II.
Step-by-step explanation:
The powerful secret weapon unleashed by the U.S. on Japan on August 6, 1945, was a nuclear weapon, known as "Little Boy". This was the world's first use of an atomic bomb in warfare, dropped on the city of Hiroshima by the U.S. Army Air Force from a B-29 Superfortress. The decision to use nuclear weapons was made by President Truman, who had been informed of the successful creation of atomic bombs through the Manhattan Project. After Japan continued the war despite the ultimatum of "prompt and utter destruction," the U.S. dropped "Little Boy" on Hiroshima and, three days later, "Fat Man" on Nagasaki. This led to the Japanese surrender on September 2, 1945.