While Einstein's theories and efforts contributed indirectly, he was not directly involved in building the world's first nuclear bomb. His theory, E=mc2, helps illustrate the energy released in an atomic reaction, but he was not responsible for instructing the United States on how to build a bomb. He did, however, warn President Roosevelt that Nazi Germany was attempting to build an atomic weapon, which influenced the United States' efforts to make their own.
In August of 1939, Einstein and a colleague sent a letter to President Franklin Roosevelt, suggesting that the United States start their own research on nuclear weapons so that they might beat the Germans to the technology. The Manhattan Project was born less than a year later, but Einstein was denied the security clearance necessary to work on the project. His political views were deemed too far left and too risky. Regardless, Einstein was pacifist and did not support building the atomic bomb altogether, but the idea of a nuclear Germany was too terrifying. When interviewed later in life, he regretted that his warning in 1939 caused the U.S. to build the bomb: "had I known that the Germans would not succeed in developing an atomic bomb, I would have done nothing."
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