During the war, Foster served as captain of the 27th Missouri Mounted Infantry until January 1862. He served as a major in the 7th Missouri Cavalry (Militia) from March 1862 until June 1864. He commanded the Union forces at the Battle of Lone Jack, in Missouri. He was wounded in this defeat by Confederate forces. Afterward, Foster was to be criticized for being outnumbered, and not waiting for reinforcements prior to his attack. He was forced to resign from the service because of his wounds.
