A declaration of war by Canada against Germany was made by order-in-council signed by George VI, King of Canada, on 10 September 1939, seven days after the United Kingdom and France had also entered a state of war with the Nazi regime. The royal proclamation of the Canadian declaration was published in the Canada Gazette.

The commander-in-chief of the Canadian Armed Forces exercises supreme command and control over Canada's military, the Canadian Armed Forces. Constitutionally, command-in-chief is vested in the Canadian monarch, presently King Charles III. Since the Letters Patent, 1947, were signed by King George VI, the governor general of Canada—presently Mary Simon—executes most of the duties of the sovereign, including in his role as commander-in-chief. Consequently, the governor general also uses the title Commander-in-Chief of the Canadian Forces. By protocol, the title used within international contexts is Commander-in-Chief of Canada.