After the reorganization of 1815, the Regular infantry fluctuated in size with the whole military establishment. Prospects of peace appeared to improve, and in 1821 Congress felt safe enough to cut expenses by disbanding the Rifle Regiment and the 8th Infantry. Having reduced the infantry establishment to seven foot regiments, which were thought adequate to meet all contingencies, the legislators next sliced the size of companies to fifty-one enlisted men, the smallest ever. This arrangement endured for fifteen years when, as usual, the Indians forced an enlargement.
There was always trouble with the Indians on the frontier, but two affairs assumed the magnitude of war. The first in 1831 and 1832 against the tribes of the Iowa, Illinois, and Wisconsin area, known as the Black Hawk War, was easily won by a force composed mostly of militia. The whole affair had no permanent impact on the Regular infantry. Not so the second of the several scraps against the Seminole Indians in Florida, beginning in December of 1835 and lasting until 1842. Volunteers and militia bore the brunt of the Florida War at first, but Regulars gradually replaced them. As a result, after more than two years of inconclusive fighting, Congress was obliged to augment the Regular infantry (in 1838) by adding thirty-eight privates and one sergeant to each company, and by raising a new 8th Infantry; the fourth unit to go by that number. At one time or another, each of the eight regiments of infantry served in the swamps of Florida.
When the war in Florida was over in 1842, all of the regiments and companies were reduced to minimum size. However, the Regular infantry actually increased. This came about because in the spring of 1843, to save money, the 2nd Dragoons were converted into a rifle regiment. They then became the first rifle corps included in the establishment for twenty years, since the Rifle Regiment had been disbanded in 1821. The horsemen, who felt degraded on foot, clung hard to their dragoon organization, but they received rifles and where, as far as is known, trained as riflemen. Efforts to remount them were continuous, and within a year they became the 2nd Dragoons again. When they were reconverted, the rifle corps disappeared once more from the Army. The President received authority from Congress to convert two or more infantry regiments into rifles if he thought it necessary, but he never exercised this authority.
In May of 1846, a new rifle unit, the Regiment of Mounted Riflemen, was established. This regiment had initially been designated for use on the Oregon Trail but was diverted at its origin into Mexican War service. Its animals were lost on the way, so only two companies, mounted on Mexican horses, acted as cavalry. The rest, armed with Model 1941 rifles, bayonets, and flintlock pistols, fought on foot.
At the start of the Mexican War, Congress tried to get along with just eight infantry regiments of Regulars, but in doing so gave the President power to expand their companies to one hundred enlisted men during the war. Ten months after hostilities commenced, it was necessary to change this policy and nine new regiments were added with the same organization as the old ones-to the Regular infantry. Eight of them, as was customary, bore numbers, the 9th through the 16th; but the other got a name. It was called the Regiment of Voltigeurs and Foot Riflemen. Half of this unit was to be mounted, the other half on foot, and each horseman was paired with a foot soldier who was to get up behind him for rapid movements. This arrangement was never executed, and the Voltigeurs became in fact a regiment of foot riflemen, armed with the same rifle (a muzzle-loader) as the Mounted Riflemen. Quite by chance, the regiment included a company of mountain howitzers and war rockets, but it was not linked with the riflemen tactically, nor were the rockets and howitzers ever used together.
Although raised as Regulars, the nine new infantry regiments created during the Mexican War were disbanded when the war was over. Their dissolution left a peace establishment of eight foot regiments. This structure seemed less adequate than it would have before 1846, for "Manifest Destiny" had entered the reckoning of the legislators. The inescapable need to protect, at least partially, the vast area taken from Mexico, and to help settlers across the great plains to California and Oregon, caused Congress to add the 9th and 10th Infantry in 1855, the fourth of both numbers in United States service. The ten regiments in existence after 1855, the 1st through the 10th, made tip the foot establishment until after the actual opening of hostilities in 1861. The Regiment of Mounted Riflemen remained active after the Mexican War, but in 1861 it was re-designated as the 3rd Cavalry.