That's an incredibly simplified view of what actually happened, and therefore a false narrative - about what you'd expect from watching a Hollywood movie. In many cases, American Indian tribes had been fighting each other for millenia, and thought they could use the newcomers to their advantage against enemy tribes. The Sioux, for example, were newcomers to the plains, having recently driven out / exterminated other tribes that had been there for quite some time.

If you're interested in this sort of history, there are many good books. Here's one:

https://www.goodreads.com/book/show/17571536-the-heart-of-ev...