The Canadian Expeditionary Force (CEF) was the expeditionary field force of Canada during the First World War. It was formed following Britain’s declaration of war on the German Empire on 15 August 1914, with an initial strength of one infantry division. The division subsequently fought at Ypres on the Western Front, with a newly raised second division reinforcing the committed units to form the Canadian Corps. The CEF and corps was eventually expanded to four infantry divisions, which were all committed to the fighting in France and Belgium along the Western Front. A fifth division was partially raised in 1917, but was broken up in 1918 and used as reinforcements following heavy casualties.

Expeditionary warfare is a military invasion of a foreign territory, especially away from established bases. Expeditionary forces were in part the antecedent of the modern concept of rapid deployment forces. Traditionally, expeditionary forces were essentially self-sustaining with an organic logistics capability and with a full array of supporting arms.