The Party was banned in May 1940 soon after the outset of World War II under the Defence of Canada Regulations of the War Measures Act. This was due to the party's antiwar policy. However, with the Nazi invasion of the Soviet Union in June 1941, the Communists reversed their line on the war and became hypersupportive of the Canadian war effort.
The Party was prohibited in May 1940 due to its pacifist views, which were enforced by the Defence of Canada Regulations under the War Measures Act. Nevertheless, with the German invasion of the Soviet Union in June 1941, the Communists reversed their stance and became fervent backers of Canada's war effort.