Colorado's earliest inhabitants included the Ute Indians, who migrated to the mountains from the Utah deserts. In the 18th century, Spaniards from New Mexico claimed the region for Spain. The state's name comes from the Colorado River, which was named by the Spanish and means "reddish colored." The United States gained control of eastern Colorado as part of the 1803 Louisiana Purchase. The first Americans in the region were soldiers led by Zebulon Pike, sent by the U.S. government in 1806 to explore. American occupation began in the early 1830s, with the completion of the Bent's Fort trading post on the Arkansas River. In 1848, after the Mexican War, the U.S. took possession of the western part of Colorado.