A more thorough explanation:

The act-of-state doctrine is a principle in international law that states that no country can judge the legality of another country's sovereign acts within its own territory. This means that a court in one country cannot make a ruling on the actions of another country's government within its own borders.

For example, if Country A passes a law that affects its citizens, a court in Country B cannot rule on the legality of that law. This is because the act-of-state doctrine prevents courts from interfering in the affairs of other countries.

The act-of-state doctrine was first established by the U.S. Supreme Court in 1897 in the case of Underhill v. Hernandez. The court stated that "the courts of one country will not sit in judgment on the acts of the government of another done within its own territory."

Although the act-of-state doctrine is not required by international law or the U.S. Constitution, it has "institutional underpinnings," according to the Supreme Court. This means that it is an important principle that helps maintain good relations between countries.