Marsh v. Seymour was a United States Supreme Court case that addressed the issue of whether a state court had the authority to issue a writ of habeas corpus to a person who was being held in custody by a federal marshal. The case arose when a federal marshal arrested a man named Seymour in the state of California for a crime committed in the state of Oregon. Seymour sought a writ of habeas corpus from the California state court, arguing that the federal marshal had no authority to arrest him in California. The Supreme Court held that the state court did not have the authority to issue a writ of habeas corpus in this case. The Court reasoned that the federal government had exclusive jurisdiction over the matter, and that the state court did not have the power to interfere with the federal government's authority. The Court also noted that the federal government had the power to enforce its laws in any state, and that the state court could not interfere with this power. The Court's decision in Marsh v. Seymour established that the federal government has exclusive jurisdiction over matters involving federal law, and that state courts cannot interfere with the federal government's authority. This decision has been cited in numerous cases since then, and has been used to support the idea that the federal government has the power to enforce its laws in any state.

In Marsh v. Seymour, the Supreme Court was asked to decide whether a religious sect known as the Shakers had legal authority to exclude non-members from their property. The majority opinion held that they did not have such authority and that any exclusion of non-members would be an unconstitutional violation of freedom of religion. Justice Field dissented, arguing that while it is true that all citizens should enjoy equal protection under the law, this case presented a unique situation in which allowing members of one particular faith group exclusive access to their own land could be seen as reasonable and necessary for preserving their beliefs and practices without infringing on anyone else's rights or freedoms. He further argued that if Congress were allowed to pass laws granting special privileges based on religion then there would be no end to what other groups might demand in terms of exemptions from general laws applicable across society - something he believed was contrary both to public policy and constitutional principles.