William Clark 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 state court. The case arose when William Clark, a prisoner in the state of Georgia, sought a writ of habeas corpus from the federal court in order to challenge his conviction in the state court. The Supreme Court held that the federal court did not have the power to issue a writ of habeas corpus to a state court. The Court reasoned that the writ of habeas corpus was a remedy that was available only to federal courts, and that the federal court did not have the power to interfere with the proceedings of a state court. The Court also noted that the writ of habeas corpus was a remedy that was available only in cases where the prisoner was being held in violation of the Constitution or laws of the United States. As such, the Court held that the federal court did not have the power to issue a writ of habeas corpus to a state court.

In the case of William Clark v. United States, the Supreme Court was asked to decide whether a federal statute that allowed for the sale of public lands in certain states applied to land owned by an individual prior to its enactment. The majority opinion held that it did not apply and thus could not be used as a basis for taking away private property without just compensation. However, Justice Swayne dissented from this decision on two grounds: first, he argued that Congress had authority under the Constitution's Property Clause to enact such legislation; second, he argued that even if Congress lacked such authority, it should still be given effect because it would promote justice and fairness between individuals who purchased land before or after passage of the law. In conclusion, Justice Swayne believed that while there may have been some constitutional issues with applying this particular law retroactively in this case, doing so would ultimately serve a greater good by providing equal protection under the law regardless of when one acquired their property rights.