A tool of the judiciary to check the powers of the executive, a writ of habeas corpus is a court order challenging the legality of a prisoner’s detention. The Judiciary Act of 1789 empowered the Supreme Court to hear habeas corpus petitions of federal prisoners and Congress expanded that jurisdiction in 1867. In 1868, when a military prisoner challenged his arrest for criticizing Reconstruction policies, Congress limited the Supreme Court’s authority, fearing the ruling’s impact on Reconstruction. The court recognized Congress’s power to limit its appellate jurisdiction in Ex parte McCardle.

The supreme Court shall have appellate Jurisdiction . . . with such Exceptions, and under such Regulations as the Congress shall make.

U.S. Constitution, Article III, Section 2