Ex parte Hagar 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 petitioner, Hagar, was a prisoner in the custody of the United States Marshal for the District of Columbia. He had been convicted of a crime in the District of Columbia and was serving his sentence in the federal prison. Hagar filed a petition for a writ of habeas corpus in the Supreme Court of the District of Columbia, seeking to be released from federal custody. The Supreme Court of the District of Columbia granted the writ, and the United States Marshal brought Hagar before the court. The United States Attorney then filed a motion to quash the writ, arguing that the state court did not have the authority to issue a writ of habeas corpus to a prisoner held in federal custody. The Supreme Court of the District of Columbia denied the motion, and the United States appealed to the Supreme Court of the United States. The Supreme Court of the United States held that the state court did not have the authority to issue a writ of habeas corpus to a prisoner held in federal custody. The Court reasoned that the writ of habeas corpus was a federal remedy, and that the state court did not have the power to interfere with the federal government's authority to detain a prisoner. The Court also noted that the writ of habeas corpus was a remedy that could only be granted by a federal court. Therefore, 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 federal custody.

In Ex Parte Hagar, the Supreme Court was asked to determine whether a state court had jurisdiction over an individual who committed a crime in another state. The majority opinion held that the defendant could be tried in the other state's court because he had been present there when he committed his offense and thus it was within their jurisdiction. However, Justice Field dissented from this decision arguing that while states have broad authority to punish crimes occurring within their borders, they do not have power to try individuals for offenses committed outside of them. He argued that such action would violate due process as well as principles of comity between states which are essential for maintaining peace and order among them. Furthermore, Justice Field noted that Congress has exclusive power under Article IV Section 2 of the Constitution to regulate interstate relations and therefore any attempt by one State’s courts to exercise criminal jurisdiction beyond its own boundaries should be considered unconstitutional unless authorized by Congress or otherwise provided for in federal law.