The Great War, later renamed World War I (WWI) after another World War came along, lasted from 1914 to 1919. The Armistice was at 11:00 AM November 11, 1918 and the fighting officially stopped. The war formally ended when the Treaty of Versailles was signed June 28, 1919.
When Britain declared war in August 1914 after German aggression, Canada was automatically at war. The law had changed before World War II occurred.
Canada committed her army in the form of new battalions, raised to create the Canadian Expeditionary Force (C.E.F.). They were numbered starting with the 1st Battalion, a battalion being about 1,000 officers and men.