The same thing that has happened for hundreds and thousands of years is still happening today. People will work and fight and die to protect their sovereignty. There is one more fundamental attribute of sovereignty that I have not mentioned yet. That is recognition by another sovereign, such as the way the United States recognizes the existence of Mexico or Canada. For Indian tribes, the mechanism of recognition was treaties.
Treaties are a contract between governments.
When two governments enter into a treaty with each other, that means they are recognizing each other as sovereigns. For example, states generally enter into contracts with each other that are called compacts. Recently, tribes have also begun to enter into compacts with states to establish casino gaming. Treaties are the basis of the relationship between tribes and the United States. When the United States government recognized tribes as sovereigns through treaties, they were following in the footsteps of European nations that had done the same thing.