If there had been a governing body of international law, the Indians could've made their case before some kind of adjudicator. Sure, the adjucator probably would've ruled against the Indians because of racial bias. But at least the Indians would've been on the record with their objections. A later adjudicator could've overturned the earlier decision on legal grounds.
Something like this has happened in the last couple centuries. The tribes have signed treaties protecting their rights. They've gone to American courts and even to UN forums demanding the US uphold their rights. They rarely win--but that's because of the US's superior political and economic power. It's not because of the merits of their cases.
In purely legal terms, untainted by outside considerations, the Indians often present an incontestable case. Namely, that the treaties are still valid. If the US obeys its own laws and Constitution, it must uphold them. Whether it actually does or not is another issue.
For more on the subject, see No Right of Return for Palestinians, Indians? and Educating Stephen About Israel's Occupation.