Reinhardt: Many of the Europeans who came to, conquered, and settled the Americas in the 16th, 17th, and 18th centuries brought with them some very heavy cultural baggage. 
They were very accomplished and there were certainly many things to admire about them, but they weren’t perfect. In general, they were intensely ethnocentric, meaning they viewed their beliefs and their ways of doing things as THE way of believing and doing. Of course, they didn’t always agree among themselves, but most of them had just about zero tolerance for foreign belief systems and life ways. To use modern terms, they were about as hostile to ideas like “diversity” and “multi-culturalism” as you could possibly get. I’m not exaggerating when I say that people like the Spanish Conquistadors of the 16th century or the English Puritans of the 17th century make Rush Limbaugh look like a flaming liberal. To them, Indians were literally Devil worshipers, 
as in they believed Indian religions to be the work of the Devil Himself. And on more secular issues, they simply considered anything that Indians did differently to be a sign of savagery and inferiority. No matter that in certain areas, such as agronomy and astronomy, Native Americans were light years ahead of Europeans. For example, when Europeans were still arguing about whether the Sun circled the Earth or the Earth circled the Sun, Indians of Central America were intricately charting the movements of heavenly bodies, and they had developed a combination lunar/solar calendar that was more accurate than the Gregorian calendar that Europeans developed and the rest of the world now uses. For many Europeans, the ethnocentric bias was so strong that it was blinding.
As Europeans and their descendants competed with Indians for control of the Western hemisphere, that competition reinforced such biases. And so in many cases, and always in what was to become the United States, it wasn’t enough to take Indians’ lands and destroy their governments. Beyond that, cultural genocide had also become a goal by the mid- to late-19th century, and it remained a central part of federal Indian policies until the 1930s. 
Even after that, it took another couple of generations for the popular, mainstream culture to start accepting Indian cultures as legitimate or even acceptable (That the pendulum swung back the other way to New Age romanticism is another issue entirely I won’t go into here). The upshot is that Americans tried very hard to destroy Indian communities and remake Indian people into individual Americans. One of the ironies, however, is that part of this attack included the creation of reservations and the sequestering of Indian populations on those reservations, which in some ways helped to strengthen Indian communities, giving them something to work with in the face of tremendous persecution. There are several hundred different reservations, each with its own history, and in fact the majority of today’s Indigenous people don’t live on them, so it’s impossible to generalize. But in some cases, strong arguments can be made that certain reservations are still communities, or maintain certain community features.