Powell v. Pennsylvania was a United States Supreme Court case that addressed the issue of whether a state could impose a death penalty for a crime that was committed before the state's death penalty statute was enacted. The case involved the conviction of a man named William Powell for the murder of his wife. Powell had been convicted in Pennsylvania and sentenced to death. Powell argued that the death penalty was unconstitutional because it was imposed after the crime was committed, and thus violated the Ex Post Facto Clause of the United States Constitution. The Supreme Court disagreed, ruling that the Ex Post Facto Clause did not apply in this case because the death penalty was not a punishment for the crime itself, but rather a deterrent to future crimes. The Court also noted that the death penalty was a long-standing practice in Pennsylvania and that the state had a legitimate interest in deterring future crimes. The Court's ruling in Powell v. Pennsylvania established that states could impose the death penalty for crimes committed before the enactment of a death penalty statute, as long as the penalty was not a punishment for the crime itself. The ruling also established that the Ex Post Facto Clause did not apply in such cases.

Justice Harlan delivered the dissenting opinion in Powell v. Pennsylvania, arguing that the majority's decision was an unjustified interference with a state's right to regulate its own affairs. He argued that while it is true that states cannot pass laws which are contrary to the Constitution or federal law, this case did not involve any such violation and thus should be left up to Pennsylvania alone. Furthermore, he noted that even if there were some constitutional issues at play here, they could have been addressed without invalidating the entire statute as unconstitutional; instead of striking down all of it, only those parts which violated constitutional rights should have been struck down. In conclusion Justice Harlan asserted his belief that this ruling would set a dangerous precedent for future cases involving similar matters and urged caution when dealing with these types of cases in order to protect both state autonomy and individual liberty from unnecessary government intrusion.