In the case of South Dakota v. Yankton Sioux Tribe et al., 1997, the U.S. Supreme Court was tasked with determining whether certain lands within Charles Mix County, South Dakota still maintained their reservation status or if they had been diminished by an 1894 Act that opened up portions of the reservation for settlement by non-Indians. The Yankton Sioux Tribe argued that these lands were part of their tribal jurisdiction and thus subject to tribal law enforcement and taxation powers, while the state contended otherwise. In a unanimous decision led by Justice David Souter, it was determined that Congress intended to diminish the boundaries when it passed this act in 1894; therefore, these areas are no longer considered Indian country under federal law but rather fall under state jurisdiction.

In the dissenting opinion for South Dakota v. Yankton Sioux Tribe et al., Justice Souter, joined by Justices Stevens and Breyer, argued that the majority's interpretation of the 1894 Act was incorrect. They contended that Congress did not intend to diminish or disestablish the Yankton Sioux Reservation but rather intended to open up a portion of it for settlement while maintaining tribal jurisdiction over these lands. The dissent pointed out that there were no clear indications in either statutory language or legislative history suggesting an intent to diminish reservation boundaries. Furthermore, they noted inconsistencies in applying precedent regarding diminishment cases and emphasized on considering all relevant evidence before concluding congressional intent about reservation boundaries alteration. Lastly, they criticized majority’s reliance on subsequent demographic changes as this could potentially allow states' encroachments upon reservations to influence judicial decisions about their legal status.