World War II and the Manhattan Project

In the summer of 1939, a few months before the beginning of World War II, Einstein was persuaded to write a letter to President Franklin D. Roosevelt and warn him that Nazi Germany might be developing an atomic bomb. The letter, dictated by Einstein,and translated with the help of Hungarian emigre physicist Leo Szilard, gave the letter more prestige, with Einstein also recommending that the U.S. begin uranium enrichment and nuclear research. According to F.G. Gosling of the U.S. Department of Energy, Einstein, Szilard, and other refugees including Edward Teller and Eugene Wigner, "regarded it as their responsibility to alert Americans to the possibility that German scientists might win the race to build an atomic bomb, and to warn that Hitler would be more than willing to resort to such a weapon.

British columnist Ambrose Evans-Pritchard notes, however, that Washington at first "brushed off with disbelief" the fears they expressed. He then describes how quickly Roosevelt changed his mind:

    Albert Einstein interceded through the Belgian queen mother, eventually getting a personal envoy into the Oval Office. Roosevelt initially fobbed him off. He listened more closely at a second meeting over breakfast the next day, then made up his mind within minutes. "This needs action," he told his military aide. It was the birth of the Manhattan Project.

Gosling adds that "the President was a man of considerable action once he had chosen a direction," and believed that the U.S. "could not take the risk of allowing Hitler" to possess nuclear bombs.
Other weapons historians agree that the letter was "arguably the key stimulus for the U.S. adoption of serious investigations into nuclear weapons on the eve of the U.S. entry into World War II". As a result of Einstein's letter, and his meetings with Roosevelt, the U.S. entered the "race" to develop the bomb first, drawing on its "immense material, financial, and scientific resources". It became the only country to develop an atomic bomb during World War II as a result of its Manhattan Project.

However, according to historian Fritz Stern, for Einstein, "war was a disease . . . [and] he called for resistance to war." But in 1933, after Hitler assumed full power in Germany, "he renounced pacifism altogether . . . In fact, he urged the Western powers to prepare themselves against another German onslaught.
In 1954, a year before his death, Einstein said to his old friend, Linus Pauling, "I made one great mistake in my life — when I signed the letter to President Roosevelt recommending that atom bombs be made; but there was some justification — the danger that the Germans would make them.