EXPEDITIONARY FORCE, a body of troops sent over seas from its home country for operations in foreign territory. The name was applied during the World War to any important force sent out to a separate theatre of war under an independent commander. Thus the British Expeditionary Force (B.E.F.), the first to proceed overseas, fought in France and Belgium, and the Mediterranean Expeditionary Force (M.E.F.) in Gallipoli, Sa lonika and Egypt ; and the United States troops in France were known as the American Expeditionary Force (A.E.F.).