Complex sentence: 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.
Split into simpler sentences: He was assigned to the western frontier under command of Gen. Henry Atkinson. In 1819, he was a member of the expedition to explore the Yellowstone River in present-day Montana and Wyoming.