Complex sentence: He went with McClellan to Washington and assisted by organizing the defense of Washington, D.C., and was promoted to colonel of volunteers that September, when he was placed in charge of the 2nd Michigan Volunteer Infantry.
Split into simpler sentences: He went with McClellan to Washington and assisted by organizing the defense of the capital. Promoted to colonel of volunteers that September, he was placed in command of the 2nd Michigan Volunteer Infantry.