Spanish conquistadores defeat a larger Aztec army at the Battle of Otumba.

The Battle of Otumba was fought between the Aztec and allied forces led by the Cihuacoatl Matlatzinctzin and those of Hernn Corts made up of the Spanish conquerors and Tlaxcalan allies, which took place on July 7, 1520, in Temalcatitln, a plain near Otumba during the development of the Conquest of the Aztec Empire. The result of the battle was a victory for the Spanish, which allowed Corts to reorganize his army, having suffered casualties a few days before in the episode known as La Noche Triste. A year later, by reinforcing his army with new men and supplies, and creating alliances with the indigenous peoples who had been subjugated by the Aztec, Corts managed to besiege and conquer Tenochtitlan.

Conquistadors (, US also ) or conquistadores (Spanish: [koŋkistaˈðoɾes], Portuguese: [kũkiʃtɐˈdoɾis, kõkiʃtɐˈðoɾɨʃ]; meaning 'conquerors') were the explorer-soldiers of the Spanish and Portuguese Empires of the 15th and 16th Centuries. During the Age of Discovery, conquistadors sailed beyond Europe to the Americas, Oceania, Africa, and Asia, colonizing and opening trade routes. They brought much of the Americas under the dominion of Spain and Portugal.

After arrival in the West Indies in 1492, the Spanish, usually led by hidalgos from the west and south of Spain, began building an American empire in the Caribbean using islands such as Hispaniola, Cuba, and Puerto Rico as bases. From 1519 to 1521, Hernán Cortés waged a campaign against the Aztec Empire, ruled by Moctezuma II. From the territories of the Aztec Empire, conquistadors expanded Spanish rule to northern Central America and parts of what is now the southern and western United States, and from Mexico sailing the Pacific Ocean to the Philippines. Other conquistadors took over the Inca Empire after crossing the Isthmus of Panama and sailing the Pacific to northern Peru. As Francisco Pizarro subdued the empire in a manner similar to Cortés other conquistadores used Peru as base for conquering much of Ecuador and Chile. Central Colombia, home of the Muisca was conquered by licentiate Gonzalo Jiménez de Quesada, and its northern regions were explored by Rodrigo de Bastidas, Alonso de Ojeda, Juan de la Cosa, Pedro de Heredia and others. For South Western Colombia, Bolivia, and Argentina, conquistadors from Peru combined parties with other conquistadors arriving more directly from the Caribbean and Río de la Plata-Paraguay respectively. All these conquests founded the basis for modern Hispanic America and the Hispanophone.

Spanish conquistadors also made significant explorations into the Amazon Jungle, Patagonia, the interior of North America, and the discovery and exploration of the Pacific Ocean. Conquistadors founded numerous cities, some of them on locations with pre-existing settlements, Manila and Mexico City.

Conquistadors in the service of the Portuguese Crown led numerous conquests for the Portuguese Empire, across South America and Africa, as well as commercial colonies in Asia, founding the origins of modern Portuguese-speaking world in the Americas, Africa, and Asia. Notable Portuguese conquistadors include Afonso de Albuquerque who led conquests across India, the Persian Gulf, the East Indies, and East Africa, and Filipe de Brito e Nicote who led conquests into Burma and was made King of Pegu.