The U.S. Supreme Court case Louisiana v. Mississippi in 1930 revolved around a territorial dispute between the two states over an area of land located on the east bank of the Mississippi River, known as South Pass. The controversy arose due to changes in the river's course which led to uncertainty about state boundaries under previous agreements and treaties dating back to colonial times. Louisiana claimed that it had jurisdiction over this territory based on historical boundary lines while Mississippi argued that its border extended up to the current main channel of the river, thus including South Pass within its domain. In deciding this case, Justice Oliver Wendell Holmes Jr., writing for a unanimous court, ruled in favor of Mississippi by applying principles from international law regarding riparian boundaries (boundaries formed by watercourses). He concluded that when a navigable river serves as a boundary and changes its course naturally over time (a process called avulsion), then existing political borders remain fixed at their original location even if they no longer coincide with geographical features like rivers or coastlines.

In the dissenting opinion for Louisiana v. Mississippi, Justice Oliver Wendell Holmes Jr. argued that the majority's decision to grant Mississippi sovereignty over a disputed area of land was incorrect due to an inaccurate interpretation of historical documents and treaties. He contended that these documents did not clearly establish Mississippi's claim as assumed by the majority, but rather suggested shared jurisdiction between both states or possibly exclusive rights for Louisiana based on its earlier settlement and development in this region. Furthermore, he criticized the court’s reliance on physical geography (the changing course of rivers) as a determinant factor in state boundaries when such changes were unpredictable and could lead to further disputes in future. In his view, legal principles should be prioritized over natural phenomena in defining territorial limits.