conquistadors

[kporter903]

why were a few thousand conquistadors able to conquer a continent of millions

[Clanad]

'Millions' may be grossly overstating the actual population of what came to be known as the South American continent (along with the Islands of Cuba, Jamaica and Hispaniola), but it was a significant conquest, regardless of the actual numbers.
There are a number of prominent reasons for the relative ease of the entrada; superior technology (the native populations had never invented the wheel, for example, much less metallurgy), the recent history of the Spaniards (and a lesser extent the Portuguese) especially the reconquista of Spain from the Moors (El Cid, etc.) wherein the Spanish learned hard lessons in warfare and developed some of their "superior technology". From something so seemingly simple as horses and war dogs, to metal armor and armorment and a few "blunderbusses".
The most significant example was Cortez' enslavement of the Aztecs ruled by Montezuma (or, more correctly Moctezuma). All of the previously mentioned aspects became focused on the Aztec empire as well as the most devastating and unexpected tool in their quiver... disease. "...Even by 1494, a Spaniard reckoned that as many as 50,000 Taino had died in and around Hispaniola, and within 20 years, their population had dwindled from 300,000, to 33,000 by 1510, and to a mere 500 by 1548..." according to The Conquest of the Americas: The Aztecs by Michael McDonnell.
The native populations simply had no defense against infestations, especially Smallpox. (Not to be outdone, the natives gave the gift of venereal disease, especially syphilis, back to their guests).
The event has been studied in depth since it occurred and there are many divergent opinions on the details, but most agree on the framework of the Conquitadores entrada (invasion)...