He was commissioned as second lieutenant in the Regiment of Mounted Rifles May 27, 1846 serving in the Mexican–American War, where he was cited for "Gallant and Meritorious conduct" in the battles of Contreras and Chapultepec.
Following the end of the war and evacuation of Mexico following the signing of the peace treaty, he took part in the 2,500-mile march of the Mounted Rifles to Oregon Territory from Missouri in 1849, where he served at Fort Vancouver from 1850–1851.
[1] Morris commanded Companies C, G, and K, Regiment of Mounted Riflemen in an engagement against a Confederate cavalry force led by Captain Bethel Coopwood, called the Skirmish near Fort Thorn, on September 26, 1861.
Coopwood was camped along the west bank of the Rio Grande, 15 miles above Fort Thorn, as they were retreating southward from their victory at the Battle of Canada Alamosa.
He was again cited for "Gallant and Meritorious conduct" for his actions in the March 31, 1865 Battle of Dinwiddie Court House in Virginia.