Sadly, there are pieces of paper with signatures on them. Its amazing how scummy the US government got in trying to "legitimize" the land forfeitures. Even sadder how some folks today want to throw out was actually signed and steal the rest.
To be fair there's some circular reasoning here. A contract itself is only valid and legal according to the law of a particular nation. Those pieces of paper had no equivalence of legal meaning in the Native American culture prior to the founding of the colonies and the forced application of English common law.
Well, a treaty is between nations and the Native American had agreements between tribes (and trade) before the Europeans showed up. The European (and later US) treaties were not a new concept, just a new form and some new, particularly scummy, tactics.