11. The Siege of Tenochtitlan, 1521
Though not one of the longest sieges in history, few had further-reaching impact on the future of the world than the Spanish led conquest of the Aztec Empire. It culminated with the fall of Tenochtitlan, the Aztec capital, following a siege from May to August. The Spanish conquistadores under Hernan Cortes were but a small part of the victorious army, which was mostly comprised of native allies, including warriors from the Tlaxcala, Texcoco, Mixquic, and other peoples long subjugated by the Aztecs. Aided by an epidemic of smallpox, which decimated the Aztec people, the siege ended with a complete Spanish victory and the establishment of an empire in the New World which catapulted Spain into a world power.
Following the collapse of the Aztec defenses and the fall of the city, the native allies took revenge by slaughtering their enemies with abandon, looting, and pillaging. The Spaniards did not interfere with the continued attacks on the Aztecs, and whole villages and towns were destroyed in the genocide which did not spare women and children. As many as a quarter of a million Aztecs, warriors, civilians, women, and children, died as a result of the conquest, while Spanish casualties were around 100. The aftermath of the siege saw the complete destruction of the Aztec culture, and the establishment of a new city on the razed remains of their capital. Today it is known as Mexico City.