In 1939, when World War II began, he resumed his work with the Ordnance Board, despite poor health, and was chosen for scientific liaison with Canada and the United States. He knew America well, having visiting professorships at Princeton and the University of Wisconsin–Madison. For this liaison work he was knighted in 1942 (see MAUD Committee). He returned to Britain later in the war and worked for the Ordnance Board and the Admiralty up until a few weeks before his death in 1944.