With the German occupation of Paris in May 1940, Halban and Kowarski left Paris with the supply of heavy water, a gramme of radium and the documentation of their research, as instructed by Joliot-Curie. He escaped France, via Clermont-Ferrand and Bordeaux, to England. He was invited by Churchill's government to continue his research at Cambridge University. In 1942, along with British and other European "refugee scientists", Halban was sent to Montreal as head of the research laboratories at the Montreal Laboratory, part of the nascent Manhattan Project.