The Aztec Empire, also known as the Triple Alliance or historiographically as the Tenochca Empire but most accurately known as the Mexica, was an alliance of three Nahua city-states: Mexico-Tenochtitlan, Tetzcoco, and Tlacopan. These three city-states ruled that area in and around the Valley of Mexico from 1428 until the combined forces of the Spanish conquistadores and their native allies who ruled under Hernán Cortés defeated them in 1521. Its people and civil society are historiographically referred to as the Aztecs or the Culhua-Mexica.