In prehistoric times, New Mexico was home to Ancestral Puebloans, Mogollon, and the modern Comanche and Utes. Spanish explorers and settlers arrived in the 16th century, naming the territory "Nuevo México" after the Aztec Valley of Mexico, more than 250 years before the establishment and naming of the present-day country of Mexico; thus, the state did "not" derive its name from Mexico. Isolated by its rugged terrain and the relative dominance of its indigenous people, New Mexico was a peripheral part of the viceroyalty of New Spain. Following Mexican independence in 1821, it became an autonomous region of Mexico, though this autonomy was increasingly threatened by the centralizing policies of the Mexican government, culminating in the Revolt of 1837; at the same time, the region became more economically dependent on the United States. At the conclusion of the Mexican–American War in 1848, the U.S. annexed New Mexico as part of the larger New Mexico Territory. It played a central role in American westward expansion, and was admitted to the Union in 1912.