Born in Bala Cynwyd, Pennsylvania, Haig served in the Korean War after graduating from the United States Military Academy. In the Korean War, he served as an aide to General Alonzo Patrick Fox and General Edward Almond. After the war, he served as an aide to Secretary of Defense Robert McNamara. During the Vietnam War, Haig commanded a battalion and later a brigade of the 1st Infantry Division. For his service, Haig was a recipient of the Distinguished Service Cross, the Silver Star with oak leaf cluster, and the Purple Heart.