Williamson v. Moore was a United States Supreme Court case that was decided in 1867. The case involved a dispute between two parties over the ownership of a slave. The plaintiff, Williamson, claimed that he had purchased the slave from Moore, the defendant, and that Moore had failed to deliver the slave to him. Moore argued that the slave had been sold to another party before the sale to Williamson had been completed. The Supreme Court ruled in favor of Williamson, finding that Moore had breached his contract with Williamson by failing to deliver the slave. The Court held that Moore was liable for damages for his breach of contract, and that Williamson was entitled to the value of the slave. The Court also held that Moore was liable for any costs incurred by Williamson in attempting to recover the slave. The decision in Williamson v. Moore established that a contract for the sale of a slave was legally binding and enforceable in the United States. The case also established that a party who breaches a contract for the sale of a slave is liable for damages and any costs incurred by the other party in attempting to recover the slave.

In the case of Williamson v. Moore, the Supreme Court was asked to decide whether a state court had jurisdiction over a dispute between two citizens of different states. The majority opinion held that it did not, as such disputes were governed by federal law and could only be decided in federal courts. However, Justice Field dissented from this decision on the basis that there was no clear indication in either statute or precedent that Congress intended for all interstate disputes to be heard exclusively in federal courts. He argued further that if Congress wanted exclusive jurisdiction over these matters they would have explicitly stated so rather than leaving it up to interpretation by judges who may disagree with one another's interpretations of congressional intent. In conclusion, he believed that allowing state courts to hear cases involving citizens from different states would help ensure justice is served without infringing upon any constitutional rights or powers granted solely to the federal government.