Complex sentence: American War (1846--1848), Hays commanded the First Regiment of Texas Rangers at the Battle of Monterrey, established six companies along the northern and western frontier of Texas, and commanded the Second Regiment of Texas Rangers in Winfield Scott's Mexico City campaign.
Split into simpler sentences: American War (1846--1848), Hays commanded the First Regiment of Texas Rangers at the Battle of Monterrey, established six companies along the northern and western frontier of Texas. He then commanded the Second Regiment of Texas Rangers in Winfield Scott's Mexico City campaign.