Declaration of war by Canada

A declaration of war by Canada is a formal declaration issued by the Government of Canada (the federal Crown-in-Council) indicating that a state of war exists between Canada and another nation. It is an exercise of the Royal Prerogative on the constitutional advice of the ministers of the Crown in Cabinet and does not require the direct approval of the Parliament of Canada, though such can be sought by the government. Since gaining the authority to declare war under the Statute of Westminster 1931, Canada has declared war only during the Second World War.

Nazi Germany
After Nazi Germany invaded Poland on September 1, 1939, the United Kingdom and France declared war on September 3. To assert Canada's independence from the UK, as already established by the Statute of Westminster 1931, Canada's political leaders decided to unnecessarily seek the approval of the federal parliament to declare war. Parliament was not scheduled to return until October 2, but returned to session early on September 7 to consider the declaration of war.

The Senate approved a declaration of war on September 8 and the House of Commons approved it on September 9. The following day, Prime Minister Mackenzie King and the Cabinet drafted an Order in Council to that effect. Canadian diplomats brought the document to King George VI, at the Royal Lodge, Windsor Great Park, for his signature, whereupon Canada had officially declared war on Nazi Germany. In his capacity as the government's official recorder for the war effort, Leonard Brockington noted: "King George VI of England did not ask us to declare war for him—we asked King George VI of Canada to declare war for us."

Fascist Italy
On June 10, 1940, Italy declared war on France and the United Kingdom. National Defence Minister Norman Rogers had been killed in a plane crash that day. Both houses of Parliament approved a declaration of war, and the Cabinet issued the Order in Council the same day.

Finland, Hungary, Romania, and Imperial Japan
Parliament adjourned on November 14, 1941, and was not scheduled to return until January 21, 1942.

At the urging of the Soviet Union, the United Kingdom declared war on Finland on December 6, 1941. At the U.K.'s urging, the Canadian Cabinet issued a proclamation declaring war on Finland, Hungary, and Romania the next day.

On December 7, 1941 (December 8 in Japan), the Empire of Japan attacked the United States at Pearl Harbor and declared war upon the United States, the United Kingdom, and Canada. Prime Minister King and the Cabinet decided to go to war with Imperial Japan that evening and issued a proclamation declaring war the following day. That same day, the U.S. and U.K. also declared war on the Japanese Empire.

When Parliament returned on January 21, 1942, King presented the Cabinet's proclamations of December 7, 1941, declaring war on Finland, Hungary, and Romania, and the Cabinet's proclamation of December 8, 1941, declaring war on Imperial Japan.