James W. Cooper served in what eventually became the 15th Northwest Arkansas Infantry. It was organized as the 3rd (McRae's) Battalion Arkansas Infantry, increased to a regiment and designated as the 21st Arkansas Infantry, and later redesignated as the 15th Northwest Arkansas Infantry.

Cooper enlisted in Co. G of this organization at Bentonville, Arkansas, on October 31, 1861; was elected third lieutenant on the same date; was promoted to second lieutenant; was not reelected at the reorganization; and was discharged on June 5, 1862.