For the last hundred years or so, we've engaged in a political fiction in which we treated the Native American Nations as sovereign, and they pretended they believed we really meant it. The Lakota Nation has chosen to call that bluff.
It happens that Aaron Two Elk, whom I mentioned recently, was Oglala Lakota. I'm sure he would be proud today. What we must watch is how the US government responds. In the past it hasn't taken movements of this type seriously; it may (and indeed will likely) simply ignore the declaration. What the Lakota Nation does in response, and what we do in response to that, will be interesting to watch.