The British declaration of war on Germany and its allies also committed the colonies and Dominions, which provided invaluable military, financial and material support. Over 2.5 million men served in the armies of the Dominions, as well as many thousands of volunteers from the Crown colonies. The contributions of Australian and New Zealand troops during the 1915 Gallipoli Campaign against the Ottoman Empire had a great impact on the national consciousness at home, and marked a watershed in the transition of Australia and New Zealand from colonies to nations in their own right. The countries continue to commemorate this occasion on Anzac Day. Canadians viewed the Battle of Vimy Ridge in a similar light. The important contribution of the Dominions to the war effort was recognised in 1917 by the British Prime Minister David Lloyd George when he invited each of the Dominion Prime Ministers to join an Imperial War Cabinet to co-ordinate imperial policy.

How many people were in the Dominions' armies?