On 6 and 9 August 1945, the United States detonated two atomic bombs over the Japanese cities of Hiroshima and Nagasaki, respectively, during the final days of World War II. The aerial bombings killed 150,000 to 246,000 people, most of whom were civilians, and remain the first and only uses of nuclear weapons in an armed conflict.

In the final year of World War II, the Allies prepared for a costly invasion of the Japanese mainland. This undertaking was preceded by a conventional bombing and firebombing campaign that devastated 64 Japanese cities, including an operation on Tokyo. The war in Europe concluded when Germany surrendered on 8 May 1945, and the Allies turned their full attention to the Pacific War. By July 1945, the Allies' Manhattan Project had produced two types of atomic bombs: "Little Boy", an enriched uranium gun-type fission weapon, and "Fat Man", a plutonium implosion-type nuclear weapon. The 509th Composite Group of the U.S. Army Air Forces was trained and equipped with the specialized Silverplate version of the Boeing B-29 Superfortress, and deployed to Tinian in the Mariana Islands. The Allies called for the unconditional surrender of the Imperial Japanese Armed Forces in the Potsdam Declaration on 26 July 1945, the alternative being "prompt and utter destruction". The Japanese government ignored the ultimatum. Japan announced its surrender to the Allies on 15 August, six days after the bombing of Nagasaki and the Soviet Union's declaration of war against Japan and invasion of Manchuria. The Japanese government signed an instrument of surrender on 2 September, ending the war.

The consent of the United Kingdom was obtained for the bombing, as was required by the Quebec Agreement, and orders were issued on 25 July by General Thomas T. Handy, the acting chief of staff of the U.S. Army, for atomic bombs to be used on Hiroshima, Kokura, Niigata, and Nagasaki. These targets were chosen because they were large urban areas that also held significant military facilities. On 6 August, a Little Boy was dropped on Hiroshima. Three days later, a Fat Man was dropped on Nagasaki. Over the next two to four months, the effects of the atomic bombings killed 90,000 to 166,000 people in Hiroshima and 60,000 to 80,000 people in Nagasaki; roughly half the deaths occurred on the first day. For months afterward, many people continued to die from the effects of burns, radiation sickness, and other injuries, compounded by illness and malnutrition. Despite Hiroshima's sizable military garrison, estimated at 24,000 troops, some 90% of the dead were civilians.

Scholars have extensively studied the effects of the bombings on the social and political character of subsequent world history and popular culture, and there is still much debate concerning the ethical and legal justification for the bombings as well as their ramifications of geopolitics especially with the context of the Cold War. Supporters argue that the atomic bombings were necessary to bring an end to the war with minimal casualties and ultimately prevented a greater loss of life on both sides, and also assert that the demonstration of atomic weaponry created the Long Peace in the interest of preventing a nuclear war. Conversely, critics argue that the bombings were unnecessary for the war's end and were a war crime, raising moral and ethical implications, and also assert that future use of atomic weaponry is more likely than anticipated and could lead to a nuclear holocaust.