This study examines the five American Legion battalions of the Canadian Expeditionary Force formed in 1915 specifically to recruit American volunteers for the Canadian overseas contingent of the First World War. This study reviews the organization of Canada's militia and Anglo-American relations before examining the formation of the American Legion, the background of its men, and the diplomatic repercussions it sparked. This study is based largely on material in the Public Archives of Canada including war records and the personal papers of several participants. During its brief existence, the American Legion precipitated constitutional, diplomatic, and political problems. The issues the American Legion raised were mostly solved by America's entry in the war. The episode hastened the maturity of Canada as a nation.