On March 3, 1847, he was brevetted 2nd lieutenant in the Second Artillery and worked as an instructor of artillery and cavalry at West Point. He served in the
Mexican-American War and was promoted to first lieutenant on December 6, 1847. He resigned his commission on July 1, 1854. as the
6th Pennsylvania Cavalry Regiment. During the
Maryland campaign, he led the third brigade of the cavalry division under brigadier general
Alfred Pleasonton. He received battle honors for service at the Battle of Hanover Court House, the Battle of Gaines' Mill, the
Battle of White Oak Swamp, the
Battle of South Mountain, and the
Battle of Antietam. He took sick leave from the regiment on April 25, 1863, due to a relapse of
malaria contracted during the Mexican-American War. Temporary command of the regiment was given to
Robert Murray Morris. Despite being on sick leave, Rush was officially listed as colonel for the regiment until September 29, 1863, when command was given to Charles L. Leiper. Rush was
seconded to the Provost Marshal General's Bureau from 10 May – 10 November 1863. and was interred at
Laurel Hill Cemetery. ==Personal life==