As with any system, there will be an expansion of a population that is only halted by the mechanisms of the system being overly stressed and unable to maintain a population over a certain level or threshold. It is natural to fight for resources in a given system. The European population settled and increased, more resources were needed, land, water, food and supplies. The Europeans fought to maintain and increase the people with whom they associated. And unfortunately a prevalent thought at the time, was the Indians were a "savage other", whose lives were not as valuable and so the Europeans eventually destroyed them.