"The Conquest" refers to the conquest of Mexico by the Spanish army led by Hernán Cortés in the early sixteenth century. In 1519, Hernán Cortés and his small army burned their ships on the coast, vowing to conquer the Aztecs. Assisted by numerous foes of the Aztecs, including the Tlaxcaltecs and Totonacs, old world diseases such as smallpox and measles, powerful weapons such as crossbows, matchlock firearms, cannon, horses, mastiffs, and superior armor, the Spanish overcame Aztec resistance by 1521. Other expeditions invaded various areas in the decades that followed, so that by the 1600s, much of what is today modern Mexico was under Spanish control. For the indigenous population, the period was a demographic disaster, with the central Valley of Mexico's population plunging from an estimated 20 million to 1.5 million by 1650.