As reported by some news outlets, Lakota activists have sent a letter to the US State Department announcing their intention to secede from the United States.
To provide context, there were several treaties between the US and the Lakota, such as the 1855 Fort Laramie treaty which granted the Lakota sovereignty, but allowed passage along the Oregon Trail - however unease between the Lakota and whites continued, following with the 1868 Fort Laramie treaty - but once gold was discovered in the Black Hills (sacred to the Lakota), all bets were off, leading to the infamous battles of Little Big Horn, where General Custer's forces were defeated, and the killing of Sitting Bull (shown on the right) at Standing Rock, and the Massacre at Wounded Knee, where 150 Lakota and 25 US soldiers were killed. The Lakota were ultimately forced to sign a treaty in 1877, ceding the Black Hills to the US, but there has been longstanding discontent over the disenfranchisement of the Lakota and loss of their sacred Black Hills.
In 1980, a proposed settlement of $122 million was offered to the Lakota by the US government, however as this did not include a return of lands, it was refused.
The potential extent of the Lakota nation is shown in the map below (current reservations shown dark red, 1868 Treaty boundary in yellow):
Wow. I'm a foreign national now. Go figure?