Pop quiz: Can U.S. citizens sue other countries in U.S. courts? Answer: Yes. There are a few exceptions to the Foreign Sovereign Immunities Act, which otherwise shields foreign countries from suits in state or federal court. Next question: Which of these exceptions did Congress create in 1996? Answer: The terrorism exception. U.S. citizens who are victims of terrorist attacks can sue a foreign state that (1) participated in or assisted the perpetrators of the attack and (2) has been designated a state-sponsor of terrorism by the U.S. State Department. Third question: Can a plaintiff suing under the terrorism exception seek punitive damages against the foreign country? Answer: Yes. Congress in 2008 listed punitive damages as a possible award for such plaintiffs. Final question: Can plaintiffs who brought a terrorism suit before 2008 still seek punitive damages? In other words, did Congress intend the punitive-damages provision to apply retroactively? Well, this was the very question the Supreme Court answered this week in Opati v. Republic of Sudan. Read on to find out.