
The military history of Canada during World War II begins with the German invasion of Poland on 1 September 1939. While the Canadian Armed Forces were eventually active in nearly every theatre of war, most combat was centred in Italy, Northwestern Europe, and the North Atlantic. In all, some 1.1 million Canadians served in the Canadian Army, Royal Canadian Navy, Royal Canadian Air Force, and in forces across the Commonwealth, with approximately 42,000 killed and another 55,000 wounded. During the war, Canada was subject to direct attack in the Battle of the St. Lawrence, and in the shelling of a lighthouse at Estevan Point in British Columbia.

The military history of Canada during World War II begins with the German invasion of Poland on 1 September 1939. While the Canadian Armed Forces were eventually active in nearly every theatre of war, most combat was centred in Italy, Northwestern Europe, and the North Atlantic. In all, some 1.1 million Canadians served in the Canadian Army, Royal Canadian Navy, Royal Canadian Air Force, and in forces across the Commonwealth, with approximately 42,000 killed and another 55,000 wounded. During the war, Canada was subject to direct attack in the Battle of the St. Lawrence, and in the shelling of a lighthouse at Estevan Point in British Columbia.

The 1943 Saint-Donat Liberator III Crash was an aerial accident that killed 24 people—the worst accident in Canadian military aviation history.

The Billion Dollar Gift and Mutual Aid were financial incentives instituted by the Canadian minister C. D. Howe during World War II.

A recommendation for a Declaration of war by Canada on Nazi Germany was announced in a speech made by Canadian Prime Minister William Lyon Mackenzie King on 3 September 1939. Though Mackenzie King was in Ottawa, Ontario, Canada, at the time of his speech, it was broadcast over the radio. There was also a Canadian announcement in the Canadian newspaper, the Canada Gazette. The declaration of war was made on 10 September 1939, just 7 days after the United Kingdom and France declared war.

Canadian propaganda during World War II(1939–45) was used to increase support for the war and commitment to an Allied victory. It was marked by the extensive use of radio and films, which had become popular since the First World War.

If Day was a simulated Nazi German invasion and occupation of the Canadian city of Winnipeg, Manitoba, and surrounding areas on 19 February 1942, during the Second World War. It was organized by the Greater Winnipeg Victory Loan organization, which was led by prominent Winnipeg businessman J. D. Perrin. The event was the largest military exercise in Winnipeg to that point.

In 1942, internment of Japanese Canadians occurred when over 22,000 Japanese Canadians, comprising over 90 percent of the total Japanese Canadian population, from British Columbia were forcibly relocated and interned in the name of national security. The majority were Canadian citizens by birth. This decision followed the events of the Japanese invasions of British Hong Kong and Malaya, the attack on Pearl Harbor in Hawaii, and the subsequent Canadian declaration of war on Japan during World War II. This forced relocation subjected many Japanese Canadians to government-enforced curfews and interrogations, job and property losses, and forced repatriation to Japan.

The Montreal Laboratory in Montreal, Quebec, Canada, was established by the National Research Council of Canada during World War II to undertake nuclear research in collaboration with the United Kingdom, and to absorb some of the scientists and work of the Tube Alloys nuclear project in Britain. It became part of the Manhattan Project, and designed and built some of the world's first nuclear reactors.

The Wartime Information Board was a Canadian government agency established on 9 September 1942, succeeding the Bureau of Public Information, to coordinate the existing public information service of the government, supervise the release from government sources of Canadian war news and information, and facilitate distribution of Canadian war information both internally and externally.