What were the immediate and long-term results of the Manhattan Project?

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Although many physicists were opposed to the actual use of the atomic bomb created by the Manhattan Project, U.S. President Harry S. Truman believed that the bomb would persuade Japan to surrender without requiring an American invasion. Accordingly, on August 6, 1945, a U.S. airplane dropped an atomic bomb on Hiroshima, killing at least 70,000 people instantly (tens of thousands more died later of radiation poisoning). Three days later, another U.S. aircraft dropped a bomb on Nagasaki. Since then, a number of countries have concluded that possession of nuclear arms is the best way to guarantee their safety, despite fears that nuclear proliferation increases the chances of the use of such a weapon.

What led to the Manhattan Project?

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In 1939, American scientists, many of whom had fled from fascist regimes in Europe, were aware of advances in nuclear fission and were concerned that Nazi Germany might develop a nuclear weapon. The physicists Leo Szilard and Eugene Wigner persuaded Albert Einstein to send a letter to U.S. President Franklin D. Roosevelt warning him of that danger and advising him to establish an American nuclear research program. The Advisory Committee on Uranium was set up in response. The beginning of the Manhattan Project can be dated to December 6, 1941, with the creation of the Office of Scientific Research and Development, headed by Vannevar Bush.