Ex parte Warmouth 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 federal custody. The case arose when a prisoner, Warmouth, was held in federal custody in Louisiana and sought a writ of habeas corpus from the state court. The state court granted the writ, and the federal government appealed the decision to the Supreme Court. The Supreme Court held that the state court did not have the authority to issue the writ of habeas corpus. The Court reasoned that the writ 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 detain a prisoner. 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 protect this right. The Court's decision in Ex parte Warmouth established that the state court did not have the authority to issue a writ of habeas corpus to a prisoner held in federal custody. The decision also established that the writ of habeas corpus was a fundamental right that the federal government had the power to protect.

In Ex Parte Warmouth, the Supreme Court was asked to decide whether a state court had jurisdiction over an action brought by a non-resident of that state. The majority opinion held that the state court did have jurisdiction in this case because it involved property located within its borders and therefore fell under its authority. However, Justice Field dissented from this decision on two grounds: firstly, he argued that since there were no allegations of fraud or illegality against the defendant in question, then it would be improper for the court to exercise personal jurisdiction over him; secondly, he argued that even if such allegations existed they should not be considered as sufficient cause for exercising personal jurisdiction unless they could be proven beyond reasonable doubt. In conclusion, Justice Field believed that due process required more evidence than what was presented before any legal action could take place against someone who resided outside of the relevant state's boundaries.