The atomic bombings of Hiroshima and Nagasaki were American bombing raids on the Japanese cities of Hiroshima and Nagasaki during World War II, which marked the first use of atomic weapons in war. Little Boy, the bomb dropped on Hiroshima, was a gun-assembly fission bomb using uranium, whereas Fat Man, the bomb dropped on Nagasaki, was an implosion fission bomb utilizing plutonium.

A number of factors contributed to the United States’ decision to drop atomic bombs on Japan. One reason was Japan’s unwillingness to surrender unconditionally. Japan wanted to keep their emperor and conduct their own war trials and did not want to be occupied by U.S. forces. However, the United States wanted unconditional surrender, which thus meant the continuation of the war. Japan refused to surrender after multiple firebombing campaigns such as the Bombing of Tokyo on March 9–10, 1945. The Bombing of Tokyo alone claimed tens of thousands of lives and is often cited as one of the most destructive acts of war in history. Although the precise death toll is unknown, conservative estimates suggest that the firestorm caused by incendiary bombs killed at least 80,000 people, likely more than 100,000, in a single night; some one million people were left homeless. It looked increasingly likely that the United States would have to commit itself to a land invasion, which could have claimed many American lives. Instead, the atomic bomb served as a tool to bring the war in the Pacific to a close sooner.

Another reason why the United States dropped the atomic bombs—and, specifically, the second one on Nagasaki—has to do with the Soviet Union. On August 8, 1945, two days after the Hiroshima bombing, as agreed to by Joseph Stalin during the Tehrān and Yalta conferences in 1943 and 1945, respectively, the Soviet Union declared war on Japan. It is possible that U.S. President Harry Truman ordered the atomic bomb to be dropped on Nagasaki not only to further force Japan to surrender but also to keep the Soviets out of Japan by displaying American military power. Distrust and a sense of rivalry had been built up between the two superpowers that ultimately culminated in the Cold War.

The atomic bombings of Hiroshima and Nagasaki produced effects in Japan and around the world that changed the course of history. Tens of thousands of people were killed in the initial explosions (an estimated 70,000 in Hiroshima and 40,000 in Nagasaki), and many more later succumbed to burns, injuries, and radiation poisoning. On August 10, 1945, one day after the bombing of Nagasaki, the Japanese government issued a statement agreeing to accept the Allied surrender terms that had been dictated in the Potsdam Declaration. The United States gained wide-reaching influence in Japan during its occupation and as a result of its installation of the Supreme Commander for the Allied Powers, a title held by the American general Douglas MacArthur. The U.S. occupation of Japan had long and lasting effects on daily life in Japan as well as on Japan’s economy, military, and government.

The atomic bombings of Hiroshima and Nagasaki also caused global effects such as the Cold War and the proliferation of nuclear weapons around the world. The Cold War was a rivalry that saw the world’s two remaining superpowers after World War II—the United States and the Soviet Union, as well as their respective allies—fight for political, economic, and nuclear superiority. Today, more countries possess nuclear weapons, but such weapons have not been used in warfare since the bombings of Hiroshima and Nagasaki.

The Soviet Union had the most nuclear weapons during the Cold War. The Soviet stockpile reached a peak of about 33,000 operational warheads in 1988, with an additional 10,000 previously deployed warheads that had been retired but not dismantled. The U.S. stockpile reached its peak in 1966 with more than 32,000 nuclear warheads.