Niels Bohr’s escape from Nazi-occupied Denmark

Niels Bohr’s escape from Nazi-occupied Denmark was a perilous passage to safety that also brought one of Europe’s leading physicists into the Allied atomic-bomb project. Smuggled out of Copenhagen in 1943, he survived a dangerous flight to Britain and then traveled to the United States, where his knowledge of the project turned escape into a new kind of responsibility: warning political leaders about the danger of a postwar nuclear arms race.

Flight from Denmark

On the night of September 29, 1943, Bohr left Copenhagen aboard a motor launch and reached the Swedish coast. He was taken to Stockholm, where German agents were plotting his assassination. The route out of Denmark had brought him beyond immediate danger, but safety remained uncertain: his rescue required another clandestine journey, this time by air.

On October 5, British airmen sent to bring him to Britain helped Bohr into the bomb bay of an unmarked Mosquito bomber. As the plywood aircraft climbed toward 20,000 feet, the pilot instructed him to put on the oxygen mask built into his leather helmet. Bohr did not hear the instruction. He later explained that the helmet was too small for his large head, and he soon fainted from lack of oxygen. He survived the flight; after landing in Scotland, he described the episode as a pleasant nap.

From Rescue to the Bomb Project

Bohr was met on the tarmac by James Chadwick, his friend and colleague, who took him to London and briefed him on the British-American atomic-bomb project. Since 1939, Bohr had understood that nuclear fission made an atomic bomb possible, but he had believed that separating uranium-235 would demand such an immense industrial effort that it was impractical. Chadwick’s account confronted him with the fact that the United States was directing its vast industrial resources toward precisely that task.

A week after Bohr arrived in London, his twenty-one-year-old son Aage joined him. Over the next seven weeks, father and son were briefed on Tube Alloys, the British code name for the bomb project. Bohr agreed to serve as a British consultant, and the British arranged for him to travel to the United States. In early December, he and Aage sailed for New York.

Arrival in the United States

Leslie Groves opposed Bohr’s participation, in part because intelligence reports portrayed the Danish physicist as difficult to contain. His prestige in the world of physics nevertheless made it hard to exclude him: Groves reluctantly authorized a visit to Site Y in the New Mexico desert. Bohr arrived at Los Alamos in December, where J. Robert Oppenheimer was enormously gratified to have him at his side.

The journey therefore carried Bohr from a narrow escape into the center of the project whose implications he had come to fear. His own earlier assumption—that the industrial demands of producing a bomb made it impractical—had been overturned by the scale of the American effort. The briefings and his visit to Los Alamos made the bomb’s development no longer a distant possibility, but an undertaking already being pursued.

The Warning Beyond the Escape

Bohr did not treat arrival in the United States as the end of his mission. A newspaper report in October had already described him in London as carrying plans for an invention involving atomic explosions, provoking Groves’s anger and efforts to restrain him. Bohr continued to approach influential figures, visiting Lord Halifax, the British ambassador, and Supreme Court Justice Felix Frankfurter in Washington.

His message was that the bomb’s development was becoming inevitable, but that its political consequences could not be postponed until after it was built. He feared a deadly nuclear arms race between the Western powers and the Soviet Union. To avert it, he urged that the Soviet Union be told that the project existed and reassured that it posed no threat. The escape thus opened onto a larger argument: survival from Nazi persecution had brought Bohr to the threshold of a weapon whose future, he believed, depended on international candor as much as on scientific achievement.