I'm guessing that they fought against forces, like the people trying to steal land so they could have enough space to live, farm, and hunt.
I also think that the Native American could have traded or given valuable items to the people who were trying to take their land in exchange for the Natives to keep their land.
In this case the word "infringed" means "<span>violated," since indeed it is restricted in the sense that there are certain firearms that people are not allowed to possess. </span>
They bought there freedom