From wiki:

The 1931 Statute of Westminster gave Canada autonomy in foreign policy. When Britain entered World War II in September 1939, some experts suggested that Canada was still bound by Britain's declaration of war because it had been made in the name of their common monarch, but Prime Minister King again said that "Parliament will decide."[11][10]: 2 

In 1936, King had told Parliament, "Our country is being drawn into international situations to a degree that I myself think is alarming."[10]: 2  Both the government and the public remained reluctant to participate in a European war, partly because of the Conscription Crisis of 1917. Both King and Opposition Leader Robert James Manion stated their opposition to conscripting troops for overseas service in March 1939. Nonetheless, King had not changed his view of 1923 that Canada would participate in a war by the Empire whether or not the United States did. By August 1939 his cabinet, including French Canadians, was united for war in a way that it probably would not have been during the Munich Crisis, although both cabinet members and the country based their support in part on expecting that Canada's participation would be "limited."[10]: 5–8 

It had been clear that Canada would elect to participate in the war before the invasion of Poland on 1 September 1939. Four days after the United Kingdom had declared war on 3 September 1939, Parliament was called in special session and both King and Manion stated their support for Canada following Britain, but did not declare war immediately, partly to show that Canada was joining out of her own initiative and was not obligated to go to war.[12] Unlike 1914 when war came as a surprise, the government had prepared various measures for price controls, rationing, and censorship, and the War Measures Act of 1914 was re-invoked.[13] After two days of debate, the House of Commons approved an Address in Reply to the Speech from the Throne on 9 September 1939 giving authority to declare war to King's government. A small group of Quebec legislators attempted to amend the bill, and CCF party leader J. S. Woodsworth stated that some of his party opposed it. Woodsworth was the only Member of Parliament to vote against the bill.[14][15] The Senate also passed the bill that day. The Cabinet drafted a proclamation of war that night, which Governor General Lord Tweedsmuir signed on 10 September.[16] King George VI approved Canada's declaration of war with Germany on Sept. 10.[17] Canada later also declared war on Italy (11 June 1940), Japan (7 December 1941), and other Axis powers, enshrining the principle that the Statute of Westminster conferred those sovereign powers on Canada.