On December 1, 1864, at the age of 25, Cyrus Walker became a private in the Oregon Infantry Volunteers of the Union Army. A year later he was promoted to first lieutenant and posted to Fort Hoskins in the Idaho Territory. His wife went with him. His primary duty was to guard the Oregon Trail, though he noted in a letter to his brother that "no one oustide has any idea how much writing it takes." Though he saw no action, he was committed to the cause and in one letter he urged his brother to sound out new recruits in Washington County to join the Union Army. At the end of the war he left the army and returned to farming, this time in Yamhill County.