He served in the Jefferson Barracks, Missouri, in 1848, carried out frontier duty at Fort Leavenworth, Kansas, in 1849, and participated in an expedition to the Rocky Mountains in 1849.

The couple had met in Kansas and, once married, lived at Fort Leavenworth, although they were separated for long periods while Steuart was on campaign duty and stationed at distant frontier posts.

[7][8] The coming of war would place considerable strain on the Steuarts' marriage, leading to "unfortunate differences", as Maria's sympathies lay firmly with the Union cause.

The Steuart mansion at Maryland Square was confiscated by the Union Army and Jarvis Hospital was erected on the estate, to care for Federal wounded.

"[5] According to Major W W Goldsborough, who served in Steuart's Maryland Infantry at Gettysburg: "...it was not only his love for a clean camp, but a desire to promote the health and comfort of his men that made him unyielding in the enforcement of sanitary rules.

[17] George Wilson Booth, a young officer in Steuart's command at Harper's Ferry in 1861, recalled in his memoirs: "The Regiment, under his master hand, soon gave evidence of the soldierly qualities which made it the pride of the army and placed the fame of Maryland in the very foreground of the Southern States".

[22] Pendleton eventually found Steuart and gave him the order to pursue Banks' retreating army but the general delayed, wasting valuable time on a point of military etiquette.

A frustrated Pendleton then rode two miles to find Ewell, who duly gave the order, but "seemed surprised that General Steuart had not gone on immediately".

Jackson continued: "There is good reason for believing that had the cavalry played its part in this pursuit, but a small portion of Banks' army would have made its escape to the Potomac".

[24] It remains unclear precisely why Steuart was reluctant to pursue Banks' defeated army more vigorously, and contemporary records shed little light on the matter.

[25] It may be that his thirteen years' training as a cavalry officer led him to obey orders to the letter, with little or no room for personal initiative or variation from strict due process.

At the Battle of Cross Keys (June 8, 1862), Steuart commanded the 1st Maryland Infantry, which was attacked by, and successfully fought off, a much larger Federal force.

At the Second Battle of Winchester (June 13–15, 1863) Steuart fought with Johnson's division, helping to bring about a Confederate victory, during which his brigade took around 1,000 prisoners and suffered comparatively small losses of 9 killed, 34 wounded.

Steuart's men arrived at Gettysburg "exhausted and footsore...a little before dusk" on the evening of July 1, following a 130-mile (210 km) march from Sharpsburg, "many of them barefooted".

[30] Steuart's men attacked the Union line on the night of July 2, gaining ground between the lower Culp's Hill and the stone wall near Spangler's Spring.

During the night a large number of Union artillery was wheeled into place, the sound of which caused the optimistic Steuart to hope that the enemy was retreating in its wagons.

The result was a "slaughterpen",[30] as the Second Maryland and the Third North Carolina regiments courageously charged a well-defended position strongly held by three brigades, a few reaching within twenty paces of the enemy lines.

[37] Steuart, bringing up the Confederate rear, halted his brigade and swiftly formed a line of battle in the road, to repel the Union attack.

[37] According to a historical marker which commemorates the engagement, Steuart's "boldness against a vastly superior force...helped to stall the advance of the entire Union army".

[5] During the battle his brother, Lieutenant William James Steuart, was severely wounded in the hip, and was sent to Guinea station, a hospital for officers in Richmond, Virginia.

[39] A friend of the family at the University of Virginia wrote to their bereaved father: Soon afterward, at the Battle of Spotsylvania Court House (May 8–21, 1864), Steuart was himself captured, along with much of his brigade, during the brutal fighting for the "Mule Shoe" salient.

The Mule Shoe salient formed a bulge in the Confederate lines, a strategic portion of vital high ground but one which was vulnerable to attack on three sides.

Exhaustion, inadequate food, lack of artillery support, and wet powder from the night's rain contributed to the collapse of the Confederate position as the Union forces swarmed out of the mist, overwhelming Steuart's men and effectively putting an end to the Virginia Brigade.

[41] During the thick of the fierce hand-to-hand fighting that followed, Steuart was forced to surrender to Colonel James A. Beaver of the 148th Pennsylvania Infantry.

[47] After the battle, Steuart was sent as a prisoner of war to Charleston, South Carolina, and was later imprisoned at Hilton Head, where he and other officers were placed under the fire of Confederate artillery.

[49] Steuart was exchanged later in the summer of 1864, returning to command a brigade in the Army of Northern Virginia, in the division of Major General George Pickett.

[48] Steuart's brigade consisted of the 9th, 14th, 38th, 53rd and 57th Virginia regiments, and served in the trenches north of the James River during the Siege of Petersburg (June 9, 1864 – March 25, 1865).

[48] By this stage of the war, Confederate supplies had dwindled to the point where Lee's army began to go hungry, and the theft of food became a serious problem.

Steuart was forced to send armed guards to the supply depot at Petersburg in order to ensure that his men's packages were not stolen by looters.

[36] He died intestate, leaving an estate valued at around $100,000, which was soon contested by Miss Grenor, who sought "pay for the 10 years she had acted as housekeeper, and also performed other duties of a more or less menial character".