According to the act of state doctrine, national courts must refrain from prosecuting the validity of official acts carried out by a foreign state within its own territory, except if it commits violations of international norms with broad consensus of international society, such as, for example, a case of genocide. Both its judicial self-restraint character and its reflection in the judicial deference to the executive branch would justify the ex officio application of the act of state doctrine by the courts. This doctrine is neither a rule nor a legal obligation required by international law, although it arises from the relevance of the international rule of territorial sovereignty of the state. It was not introduced by a constitutional or legislative provision, but is a common law principle developed mainly by Anglo-Saxon jurisdictions on the basis of considerations of international comity, respect for the principles of sovereign equality and non-intervention in the internal affairs of other states, separation of powers, and the choice of law freedom. To some extent, the legal basis of the doctrine of immunity for acts of state was analogous to the basis of immunity granted to the foreign sovereign state and its agents. The underlying rationale of this doctrine consists in preventing domestic courts from issuing adverse judgments against foreign governments that could embarrass international relations and interfere with the conduct of foreign affairs by the executive branch power. The doctrine of the act of state (and that of the political question) has important differences with the immunity of the foreign state: (1) This immunity is per se a general rule of public international law of a customary nature accepted and applied universally—in addition to being regulated in various international conventions, one of them of universal vocation—unlike the doctrines cited, which are not regulated by national legislations or by international codification efforts. (2) The moment of operation is also different, since the immunity of the foreign state functions ex ante as a procedural exception to the exercise of jurisdiction by the court of the territorial state (or court of the forum), which for that reason is obliged to recognize its lack of competence to try the case before it, while the doctrines of the state act and the political question act later—only if the defendant does not enjoy immunity—when the court is already exercising its competence and knowing the merits of the case. (3) The application of the immunity of the foreign state requires that the foreign state be sued before the courts of the forum, whereas the act of state doctrine does not require that the foreign state itself be a party to the proceedings, but it is sufficient to question the validity of an internal act of the foreign state during the judicial proceedings.