Britain had slowly, and begrudgingly, rearmed in the 1930s only after Hitler had appeared has a clear threat. Her dominions pledged their assistance to England and her allies, but not all declared war on September 3, 1939. Canada waited until September 9th to convene her Parliament, and declared war on September 10.
At the start of the war, Canada had a small Navy numbering dozens of vessels, little air forces, and no serious land forces. Her soldiers used World War I-era rifles and tactics. Her contribution to the war would be felt on every front by the end of the war.
Except for antisubmarine patrols, Canadian forces were being built up during 1939-1940, and were unable to contribute to the Allied cause in significant numbers until after the fall of France in 1940. Canadian divisions, some of the last freshly trained and equipped troops in the British Empire, were rushed to England in a perilous sea voyage that summer. It would be almost two years before they would be employed in combat at Dieppe. On June 10, 1940 Canada declared war against Italy.
Canadian pilots served in the RAF in both Fighter Command and Bomber Command during the Battle of Britain, and served with distinction. Throughout the war, Canadian losses in the CRAF bombing campaign were very heavy, as were all the air war losses.
The Canadian Navy expanded with native, British, and American construction to thousands of vessels by 1945. Canadian escort vessels braved terrible seas and weather in barely seaworthy craft that wree often slower than a surfaced U-boat to sink dozens of subs, and turn the tide of the Battle of the Atlantic in late 1943.
When Japan attacked the United States, the Royal Rifles and Winnipeg Grenadiers were in Hong Kong since November 16, 1941, and suffered hundreds of killed and wounded in combat with the invaders. Many more would die in Japanese prison camps. Canada declared war on Japan on December 8, 1941.