This dissertation examines the five battalions of the Canadian Expeditionary Force designated as the American Legion. Authorized in Canada between 1915 and 1917, these units were formed to recruit volunteers from the United States to serve in the Canadian Overseas Contingent during the First World War. This work reviews the organization of Canada’s militia and the history of Anglo-American relations before examining the Canadian war effort, the formation of the American Legion, the background of its men, and the diplomatic, political, and constitutional questions that it raised. Much of the research focuses on the internal documents of its individual battalions (the 97th, 211th, 212th, 213th and 237th) and the papers of Reverend Charles Bullock now housed at the Public Archives of Canada. Documentation for the diplomatic furor the American Legion caused comes largely through the published diplomatic documents, British Foreign Office records held at the Public Record Office at Kew, and United States Department of State files at the National Archives at College Park, Maryland. The most useful sources for American Legion correspondence are the Beaverbrook papers held at the House of Lords Record Office, the papers of Canadian Prime Minister Sir Robert Laird Borden, and those of the Governor-General, the Duke of Connaught found in the Public Archives of Canada. During its brief existence the American Legion precipitated diplomatic and political problems in the United States, the United Kingdom, and the Dominion of Canada. Among the issues raised by the controversy surrounding the American Legion were: the relationship between the dominion government in Canada and the British government; the structural problems of imperial communications; the rise of a Canadian national identity and the desire for greater autonomy; and, the nature of citizenship and expatriation. This dissertation is also a long overdue account of the thousands of United States citizens who left their homes and families to join the American Legion in order to fight another country’s war.