Within three months an entire division of the new Canadian Active Service Force had been transported to the United Kingdom, and an agreement had been announced for a British Commonwealth Air Training Plan to be centered in Canada. This project alone trained more than 131,000 aircrew personnel for the Commonwealth. Canada contributed 72,800 pilots, navigators, aerial gunners and bombardiers, and flight engineers. These Canadians saw service in almost every theater of war. The Royal Canadian Navy was increased from fewer than a dozen vessels to more than 400. It served primarily as an antisubmarine and convoy force in the North Atlantic. Some of its units were deployed from time to time as far away as the Mediterranean and the Pacific.

The forces under the command of Gen. A.G.L. McNaughton were required to spend a long and frustrating period on vital guard duty in Britain throughout the period of greatest threat of German invasion. Elsewhere abroad, two Canadian battalions sent to Hong Kong in 1941 were overrun when the colony was captured by the Japanese at the end of that year. The first engagement of the enemy by Canadian forces based in England occurred in 1942 in a courageous, but terribly costly, commando-type raid against Dieppe. In the summer of 1943 Canadian troops were sent into action with the British in the successful assault against Sicily, whence they carried the campaign to the Italian mainland.

Early in 1945 the Canadians were withdrawn from Italy to permit reunification of the Canadian Army in northwestern Europe. The climax of the war had already come, however, with the Normandy landings in June 1944, in which the Canadian Army played an important part. Instrumental in the capture of Caen, which followed, the Canadians won another major victory in the closing of the Falaise gap later the same summer. In the costly and difficult battle of the Scheldt estuary that autumn, the Canadians cleared the sea passage to Antwerp, already in Allied hands. In the bitter battle along the Hochwald Ridge in February 1945, Canadian losses were extremely heavy. This battle opened the final attack across the Rhine, which was a prelude to the unconditional surrender by Germany on May 7, 1945.

All persons over 16 years of age were required to take part in a national registration for war service, and compulsory military service for home defense only was introduced. Prime Minister King had assured the nation that there would be no conscription for overseas duty. As the war wore on, however, it became increasingly clear that the government needed to be released from the commitment. King accomplished this by a national plebiscite. All the provinces except Quebec voted in favor of conscription for overseas service if necessary. In 1944, after the Normandy invasion, the drain on manpower became so severe that draftees were sent overseas for the first time as reinforcements for the troops in Europe.

The losses in the war overseas were complemented by economic gains on the homefront. War productivity effectively ended the Great Depression and greatly increased the labor force. Canadian workers produced raw materials, farm products, and manufactured goods needed to fight the war; and this was all done in a volume unprecedented in Canadian history. Industrialization was thus rapidly advanced, through both investment of capital and striking advances in technology.