General Henry Duncan Graham Crerar was a senior officer of the Canadian Army who became the country's senior field commander in the Second World War as commander of the First Canadian Army in the campaign in North West Europe in 1944–1945. A graduate of the Royal Military College of Canada, in Kingston, Ontario, Crerar was commissioned as a lieutenant in the Non-Permanent Active Militia in 1909, serving with the 4th Battery, Canadian Field Artillery, which was based in Hamilton, Ontario. He rose to the rank of lieutenant-colonel in the artillery in the First World War, during which he was mentioned in despatches and made a member of the Distinguished Service Order (DSO). Electing to remain in the army as a professional soldier after the war, he attended the Staff College, Camberley, from 1923 to 1924, and the Imperial Defence College in 1934. He was appointed Director of Military Operations & Military Intelligence in 1935 and Commandant of the Royal Military College of Canada in 1939.

The First Canadian Army was a field army and a formation of the Canadian Army in World War II in which most Canadian elements serving in North-West Europe were assigned. It served on the Western Front from July 1944 until May 1945.