Ex parte Sawyer was a United States Supreme Court case that dealt with the issue of habeas corpus. The case involved a prisoner, William Sawyer, who was being held in a military prison in California. Sawyer had been convicted of treason and sentenced to death. He filed a petition for a writ of habeas corpus, arguing that his conviction was unconstitutional. The Supreme Court ruled in favor of Sawyer, holding that the military court did not have jurisdiction to try him for treason. The Court held that the Constitution only allowed the federal government to try individuals for treason, and that the military court had no authority to do so. The Court also held that the writ of habeas corpus was available to prisoners in military prisons, and that Sawyer was entitled to a writ of habeas corpus. The decision in Ex parte Sawyer was significant because it established the principle that the writ of habeas corpus was available to prisoners in military prisons. This decision has been cited in numerous cases since then, and has been used to protect the rights of prisoners in military prisons.

In Ex Parte Sawyer, the Supreme Court was asked to decide whether a prisoner who had been convicted of treason against the United States could be released from prison on habeas corpus. The majority opinion held that he could not, as Congress had passed a law which prohibited such releases. However, in his dissenting opinion Justice Field argued that this law violated both due process and equal protection under the Fourteenth Amendment. He reasoned that since prisoners were being denied their right to petition for release without any proof of guilt or innocence, it constituted an unconstitutional deprivation of liberty without due process of law. Furthermore, by denying certain classes of prisoners access to habeas corpus while allowing others access based solely on their crime committed (treason), it also violated equal protection rights guaranteed by the Constitution.