After the Mexican War, May was posted to several parts of the American frontier, including California, New Mexico, and Texas. He served with the First Regiment of Dragoons in the Kansas Territory during its violent abolitionist clashes. On March 3, 1855, he was promoted to major and exchanged positions with another officer to return to his old unit, the Second Dragoons. On October 27, 1855, the regiment marched to Texas, under the command of Albert Sidney Johnson.