Steen was commissioned from civilian life as a second lieutenant to the 12th U.S. Infantry in early 1847 . He served at Jefferson Barracks in St. Louis, Missouri, beginning in May 1847. By the mid-1850s, Steen was promoted to first lieutenant in the 3rd U.S. Infantry and assigned to duty at Fort Union in the New Mexico Territory, where he led several lengthy reconnaissance patrols scouting for hostile Indians. With the outbreak of the Civil War in 1861, many Missourians were forced to choose sides, especially after the state's neutrality came under test when pro-secessionist forces began organizing, and fighting between Unionists and secessionists became imminent. Steen returned to Missouri and was commissioned as the Lt Colonel of the Second Regiment, Missouri Volunteer Militia, a position he held concurrently with his commission in the U.S. Army. The Second Regiment, MVM, was composed primarily of members of the pro-secession "Minutemen" paramilitary organization. This unit was arrested by U.S. troops at Camp Jackson, on the outskirts of St. Louis on May 10, 1861, on suspicion of disloyal activities. Steen avoided arrest at Camp Jackson and reportedly submitted his resignation from the U.S. Army the same day. On June 11, 1861, Steen was appointed as a brigadier general in the secessionist Missouri State Guard under its commander, Major General Sterling Price, serving as drillmaster at the State Guard encampment at Cowskin Prairie near the Arkansas border. He also commanded the Fifth Division of the Missouri State Guard early in the war. On June 22, 1861, he received an appointment as a captain in the Regular Confederate Army's Corps of Infantry, to date to March 16, 1861. As a Missouri State Guard general, Steen led his brigade at the Battle of Wilson's Creek and the First Battle of Lexington.
Death and burial
When his Missouri State Guard brigade was sent to northern Mississippi in April 1862, he became ill at Memphis, Tennessee and returned to Missouri. In November 1862, with the Missouri troops still east of the Mississippi River, Steen was appointed colonel, commanding the 10th Missouri Infantry Regiment. He was killed during the fighting at the Battle of Prairie Grove on December 7, 1862 and his body was recovered and shipped to Fort Smith. Steen is buried in the Fort Smith National Cemetery. A memorial to Unknown Confederate Dead, made of marble, commemorates Steen, as well as Brigadier General James M. McIntosh, an Arkansan who was killed at the Battle of Pea Ridge.
