How did Niels Bohr’s view of the Allied atomic bomb project change after he reached England?

Before arriving in England in 1943, Niels Bohr was convinced that the practical difficulties of creating an atomic bomb were so significant that such a weapon could not be realized until well after the war.1

Upon his arrival in London, however, Bohr was briefed on the actual progress of the Allied project. He immediately changed his mind regarding the project's feasibility and, concerned that Germany might be pursuing a similar weapon, willingly joined the Allied effort. He subsequently made technical contributions to the design of the plutonium bomb and served as a mentor to the younger scientists at Los Alamos.1

Following his involvement, Bohr became a strong advocate for an "open world," believing that the existence of the bomb necessitated international transparency—specifically with the Soviet Union—to prevent a postwar nuclear arms race.1