The principle of separation of powers prohibits Congress from prescribing a rule of decision for the federal courts to follow in particular pending cases, because the legislative branch cannot impair the exclusive powers of another branch. Decided January 29, 1872. Opinion of the Court: Salmon P. Chase → go there Dissenting: Miller. Topic: Separation of powers. United States v. Klein, 80 U.S. 128 (1871), was a landmark United States Supreme Court case stemming from the American Civil War (1861–1865) where Chief Justice Salmon P. Chase held that a Congressional statute "impairing the effect of a pardon, and thus infringing the constitutional power of the Executive" was unconstitutional. Wiki link