 He enlisted in the regular army of the United States in 1851, joining the 2nd U.S. Cavalry, a regiment then commanded by Colonel E. V. Sumner.In this regiment young Byrnes distinguished himself in the Indian campaigns in Florida and Oregon.As the American Civil War broke out he was, on the recommendation of Colonel Sumner, commissioned as First Lieutenant in the 5th U.S. Cavalry, one of the new regiments authorized by Congress.