Niels Bohr is one of the key figures in the quantum revolution that swept science in the 20th century. His model of the atom implied a transformation of the limits of knowledge; it supplanted the mechanistic model of classical physics. This prominent proponent of the new theory defended its deepest physical and philosophical implications against skeptics like Albert Einstein. He turned his native Copenhagen into a world center of theoretical physics, although with the Nazis coming to power he was forced to leave Denmark and settle in the USA. At the end of the war, Bohr actively advocated disarmament, the internationalization of science and the peaceful use of nuclear energy.