Trusting the European invaders was their downfall in the first place. Did they really believe this agreement would be honored out of all the agreements that came before it?
Fun fact: there are several native tribes that possess independent nations within the United States. Their sovereignty is technical in some sense, but they do exist and are self-governing in many ways. I have visited more than one, and they're interesting. The history is unfortunate, but there is still a future for these people, and they're still kicking. I shall visit them again soon.