Hornsby v. United States was a United States Supreme Court case that addressed the issue of whether a federal court had the power to issue a writ of habeas corpus to a state court. The case arose when a man named Hornsby was convicted of murder in a state court in Georgia. He then filed a petition for a writ of habeas corpus in a federal court, arguing that his conviction was unconstitutional. The federal court granted the writ, and the state of Georgia appealed to the Supreme Court. The Supreme Court held that the federal court did not have the power to issue a writ of habeas corpus to a state court. The Court reasoned that the writ of habeas corpus was a remedy that was available only to federal courts, and that the federal court had no authority to interfere with the proceedings of a state court. The Court also noted that the writ of habeas corpus was a remedy that was available only in cases where the petitioner was being held in custody in violation of the Constitution. Since Hornsby was not being held in custody in violation of the Constitution, the writ of habeas corpus was not available to him. The Court's decision in Hornsby v. United States established that federal courts do not have the power to issue writs of habeas corpus to state courts. This decision has been cited in numerous subsequent cases, and it remains an important precedent in the area of federal-state relations.

In Hornsby v. United States, the Supreme Court was asked to decide whether a federal statute that prohibited citizens from carrying concealed weapons in certain states was constitutional. The majority opinion held that Congress had the power to pass such legislation under its authority granted by the Constitution's Commerce Clause and Necessary and Proper Clause. However, Justice Field dissented on this decision, arguing that Congress did not have the right to regulate activities within state borders as it would be an infringement of state sovereignty. He argued further that even if Congress could legislate in this area, there were other means available for achieving their desired result without infringing upon individual rights or encroaching upon state powers. In conclusion, he believed that while Congress may have been acting with good intentions when passing this law, it should not be allowed because it violated both individual liberties and states' rights protected by the Constitution.