United States v. Ashfield was a United States Supreme Court case that dealt with the issue of whether a federal court had jurisdiction to hear a case involving a dispute between two citizens of different states. The case involved a dispute between two citizens of different states, one of whom was a resident of the state of New York and the other a resident of the state of Pennsylvania. The dispute arose out of a contract between the two parties, in which the Pennsylvania resident agreed to pay the New York resident a certain sum of money for the purchase of a piece of land. The Pennsylvania resident failed to make the payment, and the New York resident brought suit in a federal court. The Supreme Court held that the federal court did not have jurisdiction to hear the case, as the dispute was between two citizens of different states. The Court reasoned that the dispute was a matter of state law, and that the federal court did not have the power to decide such matters. The Court further held that the federal court could not exercise jurisdiction over the dispute, as it was not a matter of federal law. The Court's decision in United States v. Ashfield established that federal courts do not have jurisdiction to hear disputes between citizens of different states. This decision has been cited in numerous subsequent cases, and has been used to support the principle that federal courts should not interfere in matters of state law.

In United States v. Ashfield, the Supreme Court was asked to decide whether a federal court had jurisdiction over an action brought by the United States against a defendant for damages resulting from his alleged violation of certain customs laws. The majority opinion held that it did not have such jurisdiction because the suit was based on state law and thus could only be brought in state court. Justice Field dissented, arguing that Congress has broad authority under Article I of the Constitution to regulate commerce with foreign nations and among states, including matters related to imports and exports; therefore, he argued that federal courts should have exclusive jurisdiction over cases involving violations of these laws regardless of their origin in state or federal law. He further noted that this interpretation would ensure uniformity across all states when dealing with issues related to interstate commerce as well as provide greater protection for citizens who are subject to conflicting interpretations between different jurisdictions regarding similar matters.