The Spanish Empire was one of the first modern global empires. It was also one of the largest empires in world history. In the 16th century Spain and Portugal were in the vanguard of European global exploration and colonial expansion and the opening of trade routes across the oceans, with trade flourishing across the Atlantic between Spain and the Americas and across the Pacific between East Asia and Mexico via the Philippines. Conquistadors deposed the Aztec, Inca and Maya governments with extensive help from local factions and laid claim to vast stretches of land in North and South America. For a time, the Spanish Empire dominated the oceans with its experienced navy and ruled the European battlefield with its fearsome and well trained infantry, the famous tercios: in the words of the prominent French historian Pierre Vilar, "enacting the most extraordinary epic in human history". Spain enjoyed a cultural golden age in the 16th and 17th centuries.
This American empire was at first a disappointment, as the natives had little to trade, though settlement did encourage trade. The diseases such as smallpox and measles that arrived with the colonizers devastated the native populations, especially in the densely populated regions of the Aztec, Maya and Inca civilizations, and this reduced economic potential of conquered areas.
In the 1520s large scale extraction of silver from the rich deposits of Mexico's Guanajuato began, to be greatly augmented by the silver mines in Mexico's Zacatecas and Bolivia's Potosí from 1546. These silver shipments re-oriented the Spanish economy, leading to the importation of luxuries and grain. They also became indispensable in financing the military capability of Habsburg Spain in its long series of European and North African wars, though, with the exception of a few years in the 17th century, Spain itself (Castile in particular) was by far the most important source of revenue. The financial burden within the peninsula was on the backs of the peasant class while the nobility enjoyed an increasingly lavish lifestyle. From the time beginning with the incorporation of the Portuguese empire in 1580 (lost in 1640) until the loss of its American colonies in the 19th century, Spain maintained the largest empire in the world even though it suffered fluctuating military and economic fortunes from the 1640s. Confronted by the new experiences, difficulties and suffering created by empire-building, Spanish thinkers formulated some of the first modern thoughts on natural law, sovereignty, international law, war, and economics; there were even questions about the legitimacy of imperialism — in related schools of thought referred to collectively as the School of Salamanca. Despite these innovations many motives for the empire were rooted in the Middle Ages. Religion played a very strong role in the spread of the Spanish empire. The thought that Spain could bring Christianity to the new world certainly played a strong role in the expansion of Spain's empire.