In the fall of 1856, Johnston was transferred to a depot for new recruits at Jefferson Barracks, Missouri. In 1857 he led surveying expeditions to determine the Kansas border. Later that year, Davis was replaced as Secretary of War by John B. Floyd, a native of Abingdon and a cousin of Johnston's by marriage. He had been a former guardian of Preston Johnston. Floyd made Johnston a brevet colonel for his actions at Cerro Gordo, a promotion that caused grumbling within the Army about favoritism. In 1859, President James Buchanan named Johnston's brother-in-law, Robert Milligan McLane, as minister to Mexico, and Johnston accompanied him on a journey to visit Benito Juárez's government in Veracruz. He was also ordered to inspect possible military routes across the country in case of further hostilities.