Even as Bohr arrived in the United States for the purpose of contributing to research on nuclear weapons, he expressed concerns for the future of nuclear weapons. He sought a way to reach the president to urge the necessity of early planning of postwar atomic policy. Soon, however, he and his son Aage, who accompanied him as an assistant and later became a theoretical physicist as well, were on their way to Los Alamos. Bohr and Oppenheimer agreed that his main responsibility should be to review all phases of the project to ensure that nothing had been overlooked. In truth, as Bohr would later tell a friend, the team did not need him to make the bomb. He did serve another vital role, however. The scientists at Los Alamos inevitably began to feel guilt, fear, and doubt about their work and the harrowing consequences to which it could lead. Bohr felt these fears as deeply as anyone, but he brought a sense of optimism to the project. He showed not only that the bomb's construction was necessary to combat Hitler, but that its power could create opportunities to establish a stronger peace than had ever been possible.

Before long, the bomb was nearing completion, and Bohr felt that it was time to make further attempts to get his warnings to the highest powers: Roosevelt and Churchill. He was less concerned about Germans than the Russians, and whether and how much they should be told. If the Americans and British were open, then international policy regulating nuclear development could be enacted. But if they hid information from the Russians, the Russians were likely to develop the knowledge themselves and then be less likely to cooperate. After many obstacles, Bohr succeeded in obtaining a meeting with Churchill, but the meeting proved unproductive, largely because the two diametrically opposed personalities could not understand each other. Churchill, who had moved many with his commanding voice, had little interest in Bohr's mild whisper.

On the other hand, Roosevelt was much more understanding. He agreed generally with Bohr's prescriptions and saw how critical his steps were in maintaining atomic energy for good purposes. Further, he agreed to attempt to change Churchill's mind at their next scheduled meeting in Quebec. Instead, Roosevelt left the meeting much less confident in Bohr's proposal. Although no record of their conversation exists, evidence indicates that Churchill may have attacked Bohr personally, questioning his credibility and insinuating his loyalty to the Russians. Churchill's charges, of course, were unfounded and could easily have been verified. Instead, Churchill relied on stubborn instinct, and the force of his personality won Roosevelt to his side.

Soon after, Truman replaced Roosevelt, and in July of 1945, the bomb was successfully tested. Ironically, Stalin showed little interest when Truman hinted at the completion of "a new weapon of unusual destructive force." The Russian premier merely expressed the hope that it would prove useful against the Japanese.

Bohr had fought a brave battle, and many scientists and statesman joined his efforts. Bohr continued campaigning even after the war, with the help of his former Copenhagen fellow Hendrik Kramers, then chairman of a United Nations committee on nuclear policy. But the arms race would develop exactly as Bohr had feared. In the meantime, as 1945 saw the emancipation of Denmark, Bohr concentrated on getting back home as soon as possible.

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