The Spanish Empire in the Americas was formed after conquering large stretches of land, beginning with Christopher Columbus in the Caribbean Islands. It comprised territories and colonies of the Spanish monarch in the Americas and the Philippines with some territory in North Africa and Oceania. In the early sixteenth century, it conquered and incorporated both the Aztec empire and Inca empire. An important element in the formation of Spain's empire was the dynastic union between Isabella I of Castile and Ferdinand II of Aragon, known as the Catholic Monarchs, which initiated political, religious, and social cohesion, but not political unification. Iberian kingdoms retained their political identities, with particular administration and juridical configurations. Although the power of the Spanish sovereign as monarch varied from one territory to another, the monarch acted as such in a unitary manner over all the ruler's territories through a system of councils: the unity did not mean uniformity. In 1580, when Philip II of Spain succeeded to the throne of Portugal (as Philip I), he established the Council of Portugal, which oversaw Portugal and its empire and "preserv its own laws, institutions, and monetary system, and united only in sharing a common sovereign." The Iberian Union remained in place until in 1640, when Portugal overthrew Hapsburg rule and reestablished independence under the House of Braganza.
Castile became the dominant kingdom in Iberia because of its jurisdiction over the overseas empire in the Americas and the Philippines, with its enormous flow of silver bullion and a slice of the Asia trade through the Philippines and Manila galleon. After a short period of delegation of authority by the crown in the Americas, the crown asserted control over those territories and established the Council of the Indies to oversee rule there. The organization of governance of the overseas empire established under the Hapsburg monarchy was significantly reformed in the late eighteenth century by the Bourbon monarchs. The Napoleonic invasion of the Iberian peninsula precipitated the Spanish American wars of independence (1808-1826), resulting the loss of its most valuable colonies. But Spain retained Cuba, Puerto Rico, the Philippines, Guam, and the Marianas, as well as various territories in Africa under Spanish rule. Following the Spanish–American War of 1898, Spain ceded its last colonies in the Caribbean and the Pacific to the United States. Its last African colonies were granted independence or abandoned during Decolonization of Africa finishing in 1976.
With the marriage of the heirs apparent to their respective thrones Ferdinand of Aragon and Isabella of Castile created a personal union that most scholars view as the foundation of the Spanish monarchy. Their dynastic alliance was important for a number of reasons, ruling jointly over a large aggregation of territories although not in a unitary fashion. They successfully pursued expansion in Iberia in the Christian Reconquest of the Muslim Kingdom of Granada, completed in 1492, for which Valencia-born Pope Alexander VI gave them the title of the Catholic Monarchs. During that military campaign, Genoese mariner Christopher Columbus sought their approval for a voyage west to find "The Indies," unexpectedly landing in the New World, giving Spain claim to vast overseas territories.
The Iberian Kingdom of Portugal had earlier retaken territory from the Muslims, completing reconquest in 1238 and settling its boundaries. Portugal then began to seek further overseas expansion, first to the port of Ceuta (1415) and then by colonizing the Atlantic islands of Madeira (1418) and the Azores (1427-1452); it also began voyages down the west coast of Africa in the fifteenth century. Its rival Castile laid claim to the Canary Islands (1402) and retook territory from the Moors in 1462. Castile and Portugal came to formal agreements over the division of new territories in the Treaty of Alcaçovas (1479), as well as securing the crown of Castile for Isabella, whose accession was challenged militarily by Portugal. Following the voyage of Christopher Columbus in 1492 and first major settlement in the New World in 1493, Portugal and Castile divided the world by the Treaty of Tordesillas (1494), which gave Portugal Africa and Asia and the Western Hemisphere to Spain. Ferdinand was particularly concerned with expansion in France and Italy, as well as conquests in North Africa.
The reign of Ferdinand and Isabella began the professionalization of the apparatus of government in Spain, which led to a demand for men of letters (letrados) who were university graduates (licenciados), of Salamanca, Valladolid, Complutense and Alcalá. These lawyer-bureaucrats staffed the various councils of state, eventually including the Council of the Indies and Casa de Contratación, the two highest bodies in metropolitan Spain for the government of the empire in the New World, as well as royal government in The Indies.