The principle that one sovereign nation voluntarily adopts or enforces the laws of another sovereign nation out of deference, mutuality, and respect.
Unlike enforcement of judgments between states in the United States (which is governed by the Comity Clause of the Constitution), there is no Constitutional obligation on a U.S. court to recognize or enforce a foreign judgment. Neither is comity of nations embodied in international law. However, sovereign nations still use comity of nations for public policy reasons.
Under comity, a reviewing court does not reopen cases that have already been heard in other courts; instead, it examines the foreign judicial system. After considering factors (such as fairness and impartiality of that foreign system, the foreign court’s personal jurisdiction over the defendant, the existence of subject matter jurisdiction, and the presence of fraud), the reviewing court might choose to respect and enforce that foreign court’s judgments.
Comity of nations is considered in forum non conveniens, antisuit injunctions, antitrust claims, conflict of laws, extraterritorial discovery, and the recognition of foreign judgments.