Hiroshima and Nagasaki: How the Atomic Bomb Changed Warfare During WWII

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  • 0:06 War in the Pacific
  • 1:57 Manhattan Project
  • 4:30 Dropping Atomic Bombs…
  • 7:32 The Aftermath
  • 8:47 Lesson Summary
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Lesson Transcript
Instructor: Alexandra Lutz

Alexandra has taught students at every age level from pre-school through adult. She has a BSEd in English Education.

As America and its WWII allies considered invading Japan, the Manhattan Project successfully developed an atomic weapon. Its use on Hiroshima and Nagasaki, Japan, precipitated VJ Day, the end of the Pacific war, on August 14, 1945.

War in the Pacific Drags On

Following the attack on Pearl Harbor, Hawaii, on December 7, 1941, it took six months for the United States to stem the tide of Japanese aggression. And for most of the next two years, Allied troops in the Pacific pushed Japan back in a series of difficult conflicts. The worst fighting started in 1944 as Japan became increasingly concerned about protecting its homeland.

In March 1945, American troops conquered Iwo Jima. Just 350 miles from Japan, the island's defenders fought nearly to the last man. A third of all U.S. Marine casualties in the entire war were at Iwo Jima. Meanwhile, American bombers were raining destruction on Japan, targeting airfields and ports and cities. After just two days, miles of city streets were leveled, a million people were left homeless and 100,000 people were dead - and that was just in Tokyo. But Japan refused to surrender. When American troops invaded Okinawa, they met even stiffer resistance. The Japanese defenders and civilians fought desperately for nearly three months, and sent in a rain of kamikaze attacks. Before it was over, they had inflicted nearly 50,000 U.S. casualties while losing as many as 200,000 of their own, but they still refused to surrender.

Meanwhile, Franklin Roosevelt died, Harry Truman became president, and Germany was defeated. In July 1945, Truman met with other Allied leaders at Potsdam to discuss next steps, including an invasion of Japan itself.

Manhattan Project

While he deliberated the planned invasion, President Truman learned about the Manhattan Project, a secret program to develop a powerful new weapon: the atomic bomb.

It began, in some ways, before America even entered the war. Upon Adolf Hitler's rise to power, Albert Einstein, the famous German Jewish physicist, had fled to the United States. And then in 1939, Einstein and other leading scientists learned that Germany was working to split a uranium atom. They quickly contacted President Roosevelt to urge him to begin a nuclear program, and thus began America's Advisory Committee on Uranium. From very humble beginnings with just $6,000 and three university labs, the program grew to include 30 sites in the U.S., Canada and Britain, with more than $2 billion in funding. In 1941, it was code-named the Manhattan Project, though its main facility was high in the mountains of Los Alamos, New Mexico. In addition to developing an atomic weapon, the program also gathered intelligence on Germany's project from behind enemy lines. Ultimately, the Manhattan Project employed over 130,000 people. Under the direction of J. Robert Oppenheimer, the massive research project was kept secret from even the vice-president. Ironically, the Soviet Union had learned about it from a well-placed spy.

After experimenting with both uranium and plutonium, as well as trying both explosion- and implosion-type weapons, the world's first nuclear bomb was detonated at the Trinity Site, in the desert of southern New Mexico. Even the scientists who had developed the bomb were shocked by its power. A mushroom cloud stretched 40,000 feet into the sky. The blinding flash was visible for 200 miles, and a shock wave blew out windows 100 miles away. The desert for half a mile was turned into glass. Civilian witnesses to the destruction were assured that an ammunition dump had exploded.

Three years of intense work had paid off. It was July 16, 1945, the day before the Potsdam Conference convened.

Dropping Atomic Bombs on Hiroshima and Nagasaki

By most accounts from the period, the decision to use the new weapon against Japan was fairly simple. The slow progress of the island invasions and Japan's refusal to surrender were becoming expensive, both in terms of dollars and casualties. If the goal was to end the war as soon as possible, why wouldn't America use every weapon at its disposal to do just that? Keep in mind that the conventional bombings in Germany and Japan up to that point were already deadly and destructive to civilian targets. Truman also justified the attack as retribution for Pearl Harbor, saying, 'When you have to deal with a beast, you have to treat him as a beast.' Finally, using this impressive new weapon was a way to send a warning message to Josef Stalin and the Soviet Union, with whom tensions were increasing quickly.

Truman approved the bombing of selected Japanese targets. But before they were dropped, the Potsdam Declaration of July 26 sent an ultimatum to Japan: surrender unconditionally or face prompt and utter destruction. The Emperor never responded.

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