1865 was also around the time the Ft. Laramie Treaty was signed/Washington DC had meetings with tribal leaders (assuming there were word 'tricks' with those documents too) https://amertribes.proboards.com/thread/2930/julius-meyer

and that's why sitting bull/natives put up such a fight, they knew their land & gold was being stolen (the 'whites' were blamed)

https://blogs.loc.gov/law/2020/06/the-battle-of-greasy-grass/