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First World War

Britain's fears of war with Germany were realised in 1914 with the outbreak of the First World War. Britain quickly invaded and occupied most of Germany's overseas colonies in Africa. In the Pacific, Australia and New Zealand occupied German New Guinea and German Samoa respectively. Plans for a post-war division of the Ottoman Empire, which had joined the war on Germany's side, were secretly drawn up by Britain and France under the 1916 Sykes–Picot Agreement. This agreement was not divulged to the Sharif of Mecca, who the British had been encouraging to launch an Arab revolt against their Ottoman rulers, giving the impression that Britain was supporting the creation of an independent Arab state.

The British declaration of war on Germany and its allies 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.

Under the terms of the concluding Treaty of Versailles signed in 1919, the empire reached its greatest extent with the addition of 1.8 million sq mi (4.7 million km) and 13 million new subjects. The colonies of Germany and the Ottoman Empire were distributed to the Allied powers as League of Nations mandates. Britain gained control of Palestine, Transjordan, Iraq, parts of Cameroon and Togoland, and Tanganyika. The Dominions themselves acquired mandates of their own: the Union of South Africa gained South West Africa (modern-day Namibia), Australia gained New Guinea, and New Zealand Western Samoa. Nauru was made a combined mandate of Britain and the two Pacific Dominions.