Why was Spain able to dominate early settlement in the Western Hemisphere?

There were a number of factors that allowed Spain to dominate the New World. With the end of the Reconquista in 1492, the Spanish were a unified nation under a strong Catholic monarchy.  The Spanish, with the voyages of Christopher Columbus, were the first Europeans in this era in the New World which gave them a major advantage.  This leverage was used to benefit the boundaries that would be drawn with Portugal in the New World in the Treaty of Tordesillas.  This treaty granted the rights to most of the important lands in the New World, lands that had not even been explored yet.  With the wealth acquired by the new lands, Spain built a powerful navy to protect its holdings in the New World.  With the introduction of new pathogens in the Americas, the Spanish found it relatively easy to subjugate the populations of indigenous peoples that inhabited the lands that it claimed.
http://www.historyworld.net/wrldhis/PlainTextHistories.asp?historyid=ab49