What made Native American peoples vulnerable to conquest by European adventurers?
The Native American peoples proved themselves inferior to the European conquerors from social, economical, political and military points of view. Many of them were in decline before the arrival of the Europeans; they overburdened the environment by hunting, depleting the fauna and flora. They also died from urban diseases, like tuberculosis and on top of this, they did not have immunity to the new European illnesses and hundreds of thousands were killed in this way. The survivors intermarried and settled in less powerful communities. The Indian population grew slowly because of the harsh conditions. On another hand, the Europeans had superior military technology; they were wearing metal armors, heavy weapons and they had cavalry. The Aztecs had the knowledge of purifying gold but did not know how to make iron tools and weapons.
Source: Henretta, James A. and David Brody. America: A Concise History, Volume I: To 1877. 4th ed., Boston: Bedford/ St. Martin’s, 2010, Chapter 1