Thomas Jefferson, in his Notes on the State of Virginia (1785), defended American Indian culture and marveled at how the tribes of Virginia "never submitted themselves to any laws, any coercive power, any shadow of government" due to their "moral sense of right and wrong".
He would later write, "I believe the Indian then to be in body and mind equal to the whiteman".
His desire was for the Native Americans to intermix with European Americans and to become one people.
To achieve that end, President Jefferson would - in addition to offering U.S. citizenship to some of the Indian nations - propose lending credit to them for trade with the expectation they would be unable to honor their debts and thereby the United States would acquire their land.
http://en.wikipedia.org/wiki/Indian_removal
There was population pressure as settlers expanded their territory, generally pushing indigenous people northward and westward. But, warfare and raiding also took place as a result of wars between European powers; in North America, they enlisted their Native American allies to help them conduct warfare against each other's settlements.
http://en.wikipedia.org/wiki/American_Indian_Wars