After the war, he was a superintendent of the United States Military Academy at West Point, New York.

He graduated from the U.S. Military Academy in 1854, third in his class of 46, and was commissioned a second lieutenant in the U.S. Army Corps of Engineers.

He participated in the Battle of Antietam, in which he was wounded while acting commander of a brigade in the 1st Division, XII Corps.

Commissioned brigadier general of volunteers in November 1862, Ruger led his brigade of the XII Corps, Army of the Potomac, in the Battle of Chancellorsville, and commanded the division of Brig.

Ruger participated in Reconstruction as the military governor of Georgia and in the Freedmen's Bureau in Alabama in 1868.