German v. United States was a Supreme Court case that addressed the issue of whether a state court could issue a writ of habeas corpus to a prisoner held in federal custody. The case arose when a prisoner, John German, was arrested by federal authorities in the state of Pennsylvania and held in federal custody. German sought a writ of habeas corpus from the state court, arguing that he was being held in violation of his constitutional rights. 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. The Court reasoned that the writ of habeas corpus was a federal prerogative 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 held that the writ of habeas corpus was a fundamental right that could not be denied to a prisoner held in federal custody. The decision in German v. United States established the principle that state courts do not have the authority to issue writs of habeas corpus to prisoners held in federal custody.

In German v. United States, the Supreme Court was tasked with determining whether a state court could try an individual for treason against the United States when that person had already been tried and acquitted in a federal court. The majority opinion held that such double jeopardy did not exist because each sovereign government (state and federal) has its own right to prosecute individuals for crimes committed within their jurisdiction. Justice Field dissented from this decision, arguing that allowing states to retry individuals who have already been acquitted by a federal court would violate the Fifth Amendment's protection against double jeopardy. He argued further that if states were allowed to do so it would create two separate systems of justice – one based on state law and another based on national law – which he believed was contrary to our nation's founding principles of equal justice under the law.