Boylan v. United States was a Supreme Court case that dealt with the issue of whether a federal court had the power to issue a writ of habeas corpus to a prisoner held in a military prison. The case arose when a prisoner, John Boylan, was held in a military prison in Washington, D.C. after being convicted of a crime in a military court. Boylan sought a writ of habeas corpus from the federal court, arguing that he was being held in violation of the Constitution. The Supreme Court held that the federal court did not have the power to issue a writ of habeas corpus to a prisoner held in a military prison. The Court reasoned that the writ of habeas corpus was a civil remedy, and that the military was a separate branch of government with its own laws and procedures. Therefore, the Court held that the federal court did not have the power to issue a writ of habeas corpus to a prisoner held in a military prison. The Court's decision in Boylan v. United States established that the federal court did not have the power to issue a writ of habeas corpus to a prisoner held in a military prison. This decision has been cited in numerous subsequent cases, and it remains an important precedent in the area of federal court jurisdiction.

In the case of Boylan v. United States, the Supreme Court was asked to decide whether a federal court had jurisdiction over an appeal from a state court decision in which damages were awarded for breach of contract. The majority opinion held that such appeals did not fall within the scope of federal judicial power and thus could not be heard by a federal court. Justice Field dissented, arguing that Congress had clearly intended to give the courts this authority when it passed legislation granting them jurisdiction over cases arising under “the laws of any State” or involving citizens from different states. He further argued that allowing appeals in these types of cases would help ensure uniformity among state decisions and prevent injustice due to conflicting interpretations between jurisdictions. In conclusion, Justice Field believed that Congress should have been given more deference on matters concerning its own legislative powers and urged his colleagues to reconsider their ruling on this issue.