In 1812 Kearny was commissioned as a First Lieutenant in the War of 1812 in the 13th Infantry Regiment. After the war, he chose to remain in the US Army. Kearny was promoted to captain on April 1, 1813; brevet major in 1823; major, 1829; and lieutenant colonel, 1833. He was assigned to the western frontier under command of Gen. Henry Atkinson, and in 1819 he was a member of the expedition to explore the Yellowstone River in present-day Montana and Wyoming.
