Nix v. Allen was a United States Supreme Court case that dealt with the issue of whether a state court could issue a writ of habeas corpus to a prisoner who was being held in a federal prison. The case arose when a prisoner, William Nix, was held in a federal prison in Arkansas. Nix sought a writ of habeas corpus from the Arkansas Supreme Court, claiming that he was being held in violation of the United States Constitution. The Arkansas Supreme Court granted the writ, and the federal government appealed to the United States Supreme Court. The Supreme Court held that the Arkansas Supreme 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 power reserved to the federal government, and that the state court did not have the authority to interfere with the federal government's power. The Court also noted that the writ of habeas corpus was a fundamental right, and that the federal government had the exclusive power to protect that right. In conclusion, the Supreme Court held that the Arkansas Supreme 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 power reserved to the federal government, and that the state court did not have the authority to interfere with the federal government's power.

In Nix v. Allen, the Supreme Court was tasked with determining whether a state court had jurisdiction to hear an appeal from a probate court's decision regarding the distribution of assets in an estate. The majority opinion held that it did not have such authority and dismissed the appeal. Justice Field dissented, arguing that under certain circumstances, states could exercise appellate jurisdiction over decisions made by probate courts. He argued that this power should be exercised when necessary to protect citizens' rights or prevent injustice due to errors committed by lower courts; otherwise, appeals would be denied on technical grounds without any consideration for justice or equity being served. Furthermore, he noted that if states were precluded from exercising their right of review in these cases then individuals may suffer irreparable harm as they would lack access to adequate legal remedies for wrongs done against them through no fault of their own.