Pre-war, The Canadian Army consisted of the Permanent Forces and the Non-Permanent Active Militia. On mobilisation at the outbreak of war, both became part of the Canadian Active Service Force.

Britain’s declaration of war did not automatically commit Canada, as had been the case in 1914. But there was never serious doubt about Canada’s response: the government and people were united in support of Britain and France. After Parliament debated the matter, Canada declared war on Germany on 10 September. Prime Minister William Lyon Mackenzie King promised that only volunteers would serve overseas. Canada was unprepared for war. The regular army of 4500 men, augmented by 51,000 partly-trained reservists, possessed virtually no modern equipment. The air force had fewer than 20 modern combat aircraft while the navy’s combat potential consisted of only six destroyers, the smallest class of ocean-going warships.

The 1 Infantry Division was activated on the 1 September 1939. The 1 Canadian Infantry Brigade was raised in Ontario. It embarked for the United Kingdom on the 17 December 1939, and arrived at Greenock on the 25 December 1939. The 2 Canadian Infantry Brigade was raised in Western Canada. It embarked for the United Kingdom on the 22 December 1939, and arrived at Greenock on the 30 December. The 3 Canadian Infantry Brigade was raised in Quebec and the Maritimes. It was the first brigade to reach the United Kingdom on the 17 December. On arrival in the U.K., it was concentrated in the Aldershot area. Following the evacuation of the bulk of the B.E.F. from Dunkirk, the 1 Canadian Infantry Brigade was sent to France in June 1940, but was withdrawn soon after landing.

The division remained in the U.K. until sailing for Sicily, where it landed on the 10 July 1943 under the command of XXX Corps. With the conclusion of the campaign in Sicily, it moved across the Straits of Messina into Italy in September 1943, now under command of XIII Corps. In March 1945, the division together with the rest of the I Canadian Corps transferred from Italy to North West Europe in order to bring all the Canadian formations together in one theatre of operations. The division disbanded in Holland on the 15 September 1945.