In 1846, after war with Mexico had begun, the mounted force was further increased. Legislation passed in May of that year to strengthen the entire Army included provision for seven regiments of cavalry manned by 12-month volunteers, a Regular regiment designated the Regiment of Mounted Riflemen, and an increase in the number of privates in each cavalry company. The Regiment of Mounted Riflemen was constituted to help establish a military road to the Oregon Territory. For a number of years the opening of the road, part of it through unexplored territory, had been discussed. Money was finally appropriated and a plan developed calling for forts from the Missouri to the Columbia. That there ought to be military protection for the project was evident, and for once a mounted force appeared to be the most economical solution.
Debates in Congress on organizing this new force brought out the point that mounted troops could be used to carry the mail, as messengers, and to guard settlers going west. One member of Congress said he would vote for raising the regiment just to restore a rifle regiment to the Army. Although the United States had once been the rifle country of the world, he contended, it had fallen behind the European nations. There was not one rifle regiment in the establishment. He further stated that the unit should be mounted because, he thought, it was idle to send infantry against Indians who would be on horseback.
Headquarters of the Regiment of Mounted Riflemen was established at Jefferson Barracks in October 1846. The companies, organized at Fort McHenry, Maryland, in Columbus, Ohio, and at Jefferson Barracks, were concentrated at the barracks by the end of the year. But, instead of going to Oregon as intended, the unit joined General Scott's force in Mexico. 'In crossing the Gulf of Mexico from New Orleans to Point Isabel, Texas, the horses were washed overboard during a storm and the regiment, except for two companies mounted on captured Mexican horses, had to fight as infantry.
The regiment was armed with the Model 1841 rifle and a flintlock pistol. Through the efforts of Capt. Samuel H. Walker of the regiment and inventor Samuel Colt, the War Department purchased 1,000 Colt single-action, 6-shot revolvers for the regiment. More than 200 of the revolvers reached Vera Cruz before the end of the war, but there is no record that the unit used them in the Mexican War campaigns.
The Mounted Riflemen, fighting dismounted at Chapultepec, earned from General Winfield Scott, Commanding General of the Army, the compliment that became their motto: "Brave Rifles! Veterans! You have been baptized in fire and blood and have come out steel."