It was not until 1855 that the U.S. Army created mounted units that were actually called cavalry. At the time of their creation they were distinct in being armed with Colt Navy pattern pistols, a new-fangled weapon for its time, but one that proved its worth during the Mexican War and on the frontier. By the outbreak of the Civil War the Regular Army had five mounted regiments: two each of dragoons and cavalry and one of mounted riflemen. In August 1861 it was decided to rename all these units as cavalry, numbered according to the seniority of the regiment. Once that was done the former 1st Dragoon Regiment became the 1st U.S. Cavalry, and what had been the First Cavalry Regiment became the 4th Cavalry.