Johnston was an engineer on the Texas-United States boundary survey in 1841; he returned to the area, appointed as chief topographical engineer of the Department of Texas, and serving from 1848 to 1853. During the 1850s he sought his previous rank, sending letters to the War Department suggesting that he should be returned to a combat regiment with his wartime rank of colonel. Secretary of War Jefferson Davis, an acquaintance of Johnston's from West Point, rebuffed these suggestions, as he did later during the Civil War, much to Johnston's irritation. Despite this disagreement, Davis thought enough of Johnston to appoint him lieutenant colonel in one of the newly formed regiments, the 1st U.S. Cavalry at Fort Leavenworth, Kansas, under Col. Edwin V. "Bull" Sumner, on March 1, 1855. (At this same time, Robert E. Lee was appointed lieutenant colonel of the 2nd U.S. Cavalry under Col. Albert Sidney Johnston (no relation). In this role, Johnston participated in actions against the Sioux in the Wyoming Territory and in the violence over slavery in the future state, known as Bleeding Kansas. He developed a mentor relationship and close friendship with one of his junior officers, Capt. George B. McClellan. Later McClellan faced him from the Union Army.