From History.com 

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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.

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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.