During his military service, Steen explored parts of the western United States including large areas of southern New Mexico and southeastern Oregon.

As a young man, Steen worked as a surveyor before joining the volunteer Mounted Rangers as a second lieutenant on 16 July 1832.

Later that year, Steen joined Colonel Stephen W. Kearny on an expedition along the Platte and then east to the Missouri River, meeting with Potawatomi Indians at Council Bluffs before returning to Fort Leavenworth.

He departed San Antonio, Texas on 29 September 1846, leading two companies of dragoons assigned to escort General John E. Wool and his staff to the front in Mexico.

[6] During the Battle of Buena Vista on 23 February 1847, Steen led a detachment of cavalry in a charge against a large body of Mexican lancers who were attempting to capture a United States Army supply train.

[7] Steen was severely wounded during the battle, earning a brevet promotion to major for personal gallantry and meritorious conduct.

[1][3][4] In the summer of 1849, Steen escorted a civilian party from Fort Leavenworth to Santa Fe in New Mexico Territory.

From there, Steen with a detachment of 50 cavalrymen tracked a band of Mimbres Apache raiders to the Santa Rita copper mines.

[1][4] In June 1850, Steen led an expedition through the Organ Mountains, around the southern edge of the White Sands desert area and into Mescalero Apache country when he ran into a superior Indian force, and he withdrew to Doña Ana.

In 1858, Steen established Fort Buchanan in the newly acquired Gadsden Purchase (near present-day Sonoita, Arizona), and became the post's first commander.

[4][8][9] Later that year, he escorted a survey party led by Lieutenant Edward Beale along a proposed road route from Fort Smith, Arkansas to the Colorado River.

He then turned south passing through the Ochoco highlands and the Trout Creek area before exploring the upper Deschutes River country.

While en route, another Army survey party led by Captain Andrew J. Smith was attacked by Indians north of Harney Lake.

To protect the survey teams in the area, Steen established Camp Union on Silver Creek, 30 miles (48 km) north of Harney Lake.

[1][10][11] On a reconnaissance expedition from Camp Union, Steen explored the country along the Donner und Blitzen River, south of Harney Lake.

On 28 September, he was promoted to lieutenant colonel, reassigned to the 2nd Cavalry Regiment, and posted to Cantonment Holt near Washington, D.C.. Steen remained there until June 1862, when he was sent to Fort Craig in the New Mexico Territory.

On 23 September 1863, he was officially retired from the Army at Benton Barracks, for incapacity resulting from long and faithful service and wounds received in the line of duty.

On 30 September 1863, he was placed in command of the Union Army's fort at Sandy Hook, New Jersey, overlooking New York Harbor.

[1][3][15] When Congress passed a law prohibiting retired officers from serving on active duty, Steen left the Army.

[1][3] Steen played an important part in the history of the western United States, leading military expeditions through large areas of the west.