The United States becomes the first and only nation to use atomic weaponry during wartime when it drops an atomic bomb on the Japanese city of Hiroshima. Though the dropping of the atomic bomb on Japan marked the end of World War II, many historians argue that it also ignited the Cold War.

Since 1940, the United States had been working on developing an atomic weapon, after having been warned by Albert Einstein that Nazi Germany was already conducting research into nuclear weapons. By the time the United States conducted the first successful test (an atomic bomb was exploded in the desert in New Mexico in July 1945), Germany had already been defeated. The war against Japan in the Pacific, however, continued to rage. President Harry S. Truman, warned by some of his advisers that any attempt to invade Japan would result in horrific American casualties, ordered that the new weapon be used to bring the war to a speedy end. On August 6, 1945, the American bomber Enola Gay dropped a five-ton bomb over the Japanese city of Hiroshima. A blast equivalent to the power of 15,000 tons of TNT reduced four square miles of the city to ruins and immediately killed 80,000 people. Tens of thousands more died in the following weeks from wounds and radiation poisoning. Three days later, another bomb was dropped on the city of Nagasaki, killing nearly 40,000 more people. A few days later, Japan announced its surrender.

In the years since the two atomic bombs were dropped on Japan, a number of historians have suggested that the weapons had a two-pronged objective. First, of course, was to bring the war with Japan to a speedy end and spare American lives. It has been suggested that the second objective was to demonstrate the new weapon of mass destruction to the Soviet Union. By August 1945, relations between the Soviet Union and the United States had deteriorated badly. The Potsdam Conference between U.S. President Harry S. Truman, Russian leader Joseph Stalin, and Winston Churchill (before being replaced by Clement Attlee) ended just four days before the bombing of Hiroshima. The meeting was marked by recriminations and suspicion between the Americans and Soviets. Russian armies were occupying most of Eastern Europe. Truman and many of his advisers hoped that the U.S. atomic monopoly might offer diplomatic leverage with the Soviets. In this fashion, the dropping of the atomic bomb on Japan can be seen as the first shot of the Cold War. If U.S. officials truly believed that they could use their atomic monopoly for diplomatic advantage, they had little time to put their plan into action. By 1949, the Soviets had developed their own atomic bomb and the nuclear arms race began.