King vs. Ackerman was a case heard by the United States Supreme Court in 1862. The plaintiff, King, had purchased land from Ackerman and then discovered that it was part of an Indian reservation. He sued for breach of contract on the grounds that he would not have bought the land if he had known it belonged to Indians. The court held that since there were no laws prohibiting such sales at the time, King's claim could not be sustained as a matter of law and his suit failed. This decision established precedent which allowed non-Indians to purchase lands within reservations without fear of legal repercussions so long as they did not violate any existing laws or treaties with Native American tribes at the time of sale.

In King v. Ackerman, the Supreme Court was asked to decide whether a state court had jurisdiction over a case involving two citizens of different states. The majority opinion held that it did not have such jurisdiction and dismissed the appeal. In his dissent, Justice Nelson argued that under Article III of the Constitution, Congress has granted federal courts exclusive original jurisdiction in cases where one party is from another state or country. He further noted that this power could only be taken away by an act of Congress and no such act existed in this case; thus he concluded that the state court had proper authority to hear the dispute between these two parties from different states. Ultimately, Justice Nelson believed there was no reason why a citizen should not be able to seek justice through their own local courts when they are involved in disputes with other citizens outside their home state's borders.