United States ex rel. Phillips v. Gaines was a United States Supreme Court case that dealt with the issue of habeas corpus. The case involved a man named William Phillips, who was convicted of murder in the state of Missouri and sentenced to death. Phillips filed a petition for a writ of habeas corpus, arguing that his conviction was unconstitutional because he had not been given a fair trial. The Supreme Court agreed with Phillips, ruling that the state of Missouri had violated his right to due process of law by denying him a fair trial. The Court held that the writ of habeas corpus was available to Phillips, and that he should be released from custody. The decision in this case established the principle that the writ of habeas corpus is available to individuals who have been denied a fair trial. This case also established the principle that the writ of habeas corpus is available to individuals who have been denied due process of law. This case is an important precedent in the area of habeas corpus and due process of law.

In United States Ex Rel. Phillips v. Gaines, the Supreme Court was tasked with determining whether a writ of habeas corpus should be issued to release an individual from unlawful imprisonment in Missouri. The majority opinion held that the writ should not be granted because it would interfere with state sovereignty and judicial proceedings already underway in Missouri courts. Justice Field dissented, arguing that federal law allowed for such a writ to issue when necessary and proper under certain circumstances; he argued that this case met those criteria as there were no other legal remedies available for the petitioner's situation at hand. He further noted that allowing states to imprison individuals without due process or access to justice violated fundamental principles of liberty enshrined in both federal and state constitutions, making it essential for Congress to provide relief through habeas corpus when appropriate.