He was commissioned a second lieutenant in the 7th U.S. Infantry and stationed in Arkansas and the Indian Territory for most of his United States Army career.

[2] Brigadier General Pearce assumed command of the 1st (Western) Division and had the following units under his direct command:[3] Brigadier General Pearce, who lived in Benton County, established the headquarters, 1st Division, Provisional Army of Arkansas at Camp Walker at Maysville.

Thus when a Union army began operating around Springfield in Southwest Missouri, Pearce's state troops were nearby.

On August 10, 1861, Brigadier General Nathaniel Lyon, the forceful commander of Union troops in Missouri, attacked the Confederates.

Colonel John Gratiot's 3rd Arkansas Infantry, State Troops suffered 109 casualties, including 25 killed, out of a force of 500 men.

[4][6] Pearce's brief and contentious leadership came to an end shortly after the battle, when in August, Arkansas officials sought to transfer his brigade to Confederate service.

[8] From December 13, 1861, to the end of the war, Pearce served as a major in the Confederate Commissary Department in Arkansas, the Indian Territory, and Texas.