How Can Autonomous Indian Nations Exist Within The United States?

Between 1790 and 1871, the fledgling United States needed land to pay off massive debts. One strategy was to institute treaties with Indian groups to purchase their vast holdings.
In exchange the federal government promised peace and sovereignty. Tribes also relinquished certain rights of self-governance in exchange for promises.
The U.S. Constitution recognizes tribes as distinct governments with their own laws. Reservations (or "nations") are not subject to the government of the states that contain them. The paradox, however, is that Congress can, ultimately, limit sovereign powers.
Indians' dependency on the government has created serious social problems. They have the highest suicide and alcoholism rate, one of the worst health and obesity levels, and one of the highest poverty rates among U.S. ethnic groups.