Cocks v. Izard was a United States Supreme Court case that dealt with the issue of whether a state court had the authority to issue a writ of habeas corpus to a prisoner held in a federal prison. The case arose when a prisoner, John Cocks, was held in a federal prison in the District of Columbia. Cocks sought a writ of habeas corpus from the Supreme Court of the District of Columbia, which was denied. Cocks then appealed to the Supreme Court of the United States. The Supreme Court held that the state court did not have the authority to issue a writ of habeas corpus to a prisoner held in a federal prison. The Court reasoned that the power to issue writs of habeas corpus was a federal power, and that the state court did not have the authority to interfere with the federal government's power to imprison individuals. The Court also noted that the writ of habeas corpus was a fundamental right, and that the state court should not be allowed to interfere with the federal government's power to imprison individuals. The Court's decision in Cocks v. Izard established that state courts do not have the authority to issue writs of habeas corpus to prisoners held in federal prisons. This decision has been cited in numerous subsequent cases, and has been used to support the idea that the federal government has the power to imprison individuals without interference from the states.

In Cocks v. Izard, the Supreme Court was asked to decide whether a state court had jurisdiction over an action brought by a citizen of one state against another in which the amount in controversy exceeded $500. The majority opinion held that it did not have such jurisdiction and dismissed the case. However, Justice Field dissented from this decision on two grounds: first, he argued that under Article III of the Constitution, Congress has exclusive power to regulate cases between citizens of different states; second, he maintained that since Congress had not acted on this issue yet, it should be left for them to decide rather than being decided by judicial interpretation. He concluded his dissent with a strong statement about how important it is for federal courts to protect citizens’ rights when they are sued across state lines and urged Congress to act quickly so as not to leave individuals without recourse if their rights were violated outside their home states.