The US didn't break the treaty first (or even second). The truth is that there isn't clear pedigree to that land near as I can tell. I've tried to figure out title as far back as 2004. Beyond that, I'd probably have to go to Morton County to figure out more. It's very likely that if you go back far enough, it was transferred under the Dawes Act. Whatever the pedigree, the venue to deal with this dispute was in the courts well before 2014. The courts are the right venue to determine who owns the land, not the court of public opinion. The entire conflict happening there is an exercise in virtue signaling by environmentalists that are using another cause to further theirs. It's populism and vigilantism rolled into one. Institutions like the US Justice system actually work really well by global standards. Allowing disputes to negotiated outside the court system adds lots of legal uncertainty that destabilizes day to day life.

FWIW I wasn't even born in the United States, but I am an immigrant that has been here since 1986.