How did western expansion affect the size of the U.S. Army before the Mexican-American War?

Before the Mexican-American War, western expansion and frontier conflicts directly influenced the size of the U.S. Army, though the force remained small and subject to frequent fluctuations based on immediate threats.1

Following the War of 1812, the army was reduced to 10,000 men and further decreased to 6,127 by 1821. However, as the nation expanded westward, the military faced new pressures. By 1838, the combination of the Second Seminole War and the need to manage the growing western frontier led Congress to authorize an increase in the army's strength to 12,577.1

Once the Second Seminole War concluded in 1842, the army was again reduced to 8,613 men, a level it maintained until the outbreak of the Mexican-American War in 1846. Throughout this period, the army was tasked with occupying over 100 posts across the country, reflecting the logistical challenges of maintaining a presence in an increasingly vast territory.1