A/In 1847, De Russy was commissioned as a second lieutenant in the US Army and served in the Mexican–American War, first winning a brevet to 1st Lieutenant for "gallantry and meritorious service" at the Battle of Contreras and the Churubusco, and later, a brevet to captain for "gallant and meritorious service" at Chapultepec.[7] By virtue of his service in the Mexican War, he became a member of the Aztec Club of 1847 in 1881.[8][7]He remained in the army following the war, serving as quartermaster at Fort Monroe from 1848 until 1857.[9] That year he was promoted to captain.[8]By the start of the Civil War, De Russy was serving in the 4th US Artillery. He commanded the artillery reserve of III Corps in the Peninsula Campaign and the Seven Days Battles, receiving promotions to major and lieutenant colonel.[10] He was promoted to colonel in March 1863 and again to brigadier general of volunteers in May of the same year. He commanded the southern defenses of Washington, D.C. until the end of the war.[8]After the war, De Russy reverted to the rank of major as the forces were reduced. He continued serving in the army, receiving new promotions to lieutenant colonel in 1879, and colonel in 1882, until retiring in 1882.[8]B