Hernan Cortes was a Spanish conquistador who is best known for leading the expedition that caused the fall of the Aztec Empire in Mexico. He arrived in Mexico in 1519 and, with the help of indigenous allies and superior military tactics, he was able to defeat the Aztec ruler Montezuma and take control of the Aztec capital, Tenochtitlan, in 1521. Cortes' conquest of the Aztec Empire played a significant role in the Spanish colonization of the Americas.