Ex parte Parker is a United States Supreme Court case that dealt with the issue of whether a state court had the authority to issue a writ of habeas corpus to a prisoner held in federal custody. The case arose when a prisoner, William Parker, was held in federal custody in the state of Mississippi. Parker sought a writ of habeas corpus from the state court, claiming that he was being held in violation of the Constitution. The state court granted the writ, and the federal government appealed the decision to the Supreme Court. 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 power, and that the state court did not have the authority to interfere with the federal government's power to detain prisoners. The Court also noted that the writ of habeas corpus was a fundamental right, and that the state court should not be allowed to interfere with the federal government's power to protect this right. In conclusion, 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 power, and that the state court did not have the authority to interfere with the federal government's power to detain prisoners.

In Ex Parte Parker, the Supreme Court was asked to decide whether a writ of habeas corpus should be issued in order for an individual to challenge his confinement. The majority opinion held that the writ should not issue because it would interfere with state proceedings and undermine public confidence in judicial decisions. Justice Field dissented from this decision, arguing that federal courts have jurisdiction over matters involving civil rights and liberties, including those related to imprisonment or detention. He argued that if a person is unlawfully detained by state authorities then they are entitled to relief through a federal court's issuance of the writ of habeas corpus. Furthermore, he noted that there is no reason why such relief could not be granted without interfering with any pending state proceedings or undermining public confidence in them.