The Communist Party was banned in 1941 under the Defence of Canada Regulations and Buck and other prominent Communist leaders were forced underground and ultimately into exile in the United States because of their support for the pact between Hitler and the USSR; both parties mutually invaded Poland which sparked the Second World War. Like supporters and immigrants from Axis countries, communists were considered in collusion with Hitler and all suspected of strongly supporting this alliance were interned under the War Measures Act. The political environment only began to change with the German invasion of the USSR and the Soviet Union's entry into World War II on the side of the Allies. As a result, Canadian Communists ended their opposition to the war and apparently became enthusiastic supporters of the Canadian war effort. The party supported the government's call for conscription and established Tim Buck Plebiscite Committees which called for a "Yes" vote in the 1942 national plebiscite on conscription. The campaigning in support of the war helped change public opinion towards the Communists and resulted in the government's release of Communist leaders being held in detention and the return of Buck and other leaders from exile. While the ban on the party itself was not lifted it was allowed to organize the Labor-Progressive Party as a legal public face.