But the rise of Nazism forced him to rethink his pacifism. Einstein was out of the country when Hitler took power in 1933. He never again set foot in his homeland. Einstein and his second wife, Elsa, settled in Princeton, New Jersey, where he accepted a position at the Institute for Advanced Study.
Einstein, who was concerned that Germany was developing an atomic bomb, wrote to President Franklin Roosevelt urging that the United States accelerate its own work on nuclear weapons to deter the Germans from using any they might develop. Two years later, the United States embarked on the Manhattan Project, its effort to build a nuclear bomb. Though it was Einstein's equation E=mc2 that had shown that the tiny mass of an atom could be converted into a powerful destructive energy, Einstein was never invited to work on the project. The U.S. government distrusted him because of his left–leaning politics‹by the time Einstein died, the FBI had a nearly 1,500–page file on him. Einstein never expected the atomic bomb to be used. He was horrified when the United States dropped two atomic bombs on Japan, which killed hundreds of thousands of people. "Woe is me," he said, after the first bomb fell on Hiroshima.
Though Einstein had long used his fame to promote peace and human rights, in the years after World War II, he became even more fervently politically active. He worked tirelessly for nuclear disarmament and spoke out against racism and McCarthyism. Despising nationalism, he promoted international cooperation and the work of the United Nations. He also supported the establishment of the State of Israel. In 1952, the Israeli ambassador to the United States suggested that he become the president of Israel, an offer he politely declined.
Einstein lived as quietly as possible in Princeton, given his celebrity. He had become a beloved figure, an expert on everything. He answered as many of the deluge of letters he received as he could, whether they were about science, politics, or a child's homework, and he surprised his steady stream of visitors with his self–deprecating good humor. Still, Einstein was uncomfortable with his fame. He told one reporter, "In the past it never occurred to me that every casual remark of mine would be snatched up and recorded. Otherwise I would have crept further into my shell."
During the last decades of his life, Einstein tried to uncover what is known as the Grand Unified Theory, a theory that can describe the entire physical world. Such a theory would connect all branches of physics, it would explain everything. "I want to know how God created this world," Einstein once remarked. "I am not interested in this or that phenomenon, I want to know His thoughts; the rest are details." Einstein never succeeded in coming up with a Grand Unified Theory, and it remains one of the burning questions for theoretical physicists.
Albert Einstein died from a burst blood vessel on April 18, 1955, in a Princeton hospital. Doctors had suggested surgery, but Einstein declined. "It is tasteless to prolong life artificially," he said. "I have done my share; it is time to go. I will do it elegantly."