In Public International Law, when a state or international agency wishes to plead sovereign or diplomatic immunity in a foreign court, it requests the Foreign Office of the state where it is sued to convey to the court that said defendant is entitled to immunity.
In the United States, the procedure followed is the process of "suggestion," where the foreign state or the international organization sued in an American court requests the Secretary of State to make a determination as to whether it is entitled to immunity. If the Secretary of State finds that the defendant is immune from suit, he, in turn, asks the Attorney General to submit to the court a "suggestion" that the defendant is entitled to immunity. In England, a similar procedure is followed, only the Foreign Office issues a certification to that effect instead of submitting a "suggestion" (O'Connell, I International Law 130 [1965]; Note: Immunity from Suit of Foreign Sovereign Instrumentalities and Obligations, 50 Yale Law Journal 1088 [1941]).
In the Philippines, the practice is for the foreign government or the international organization to first secure an executive endorsement of its claim of sovereign or diplomatic immunity. But how the Philippine Foreign Office conveys its endorsement to the courts varies. In International Catholic Migration Commission v. Calleja, 190 SCRA 130 (1990), the Secretary of Foreign Affairs just sent a letter directly to the Secretary of Labor and Employment, informing the latter that the respondent-employer could not be sued because it enjoyed diplomatic immunity. In World Health Organization v. Aquino, 48 SCRA 242 (1972), the Secretary of Foreign Affairs sent the trial court a telegram to that effect. In Baer v. Tizon, 57 SCRA 1 (1974), the U.S. Embassy asked the Secretary of Foreign Affairs to request the Solicitor General to make, in behalf of the Commander of the United States Naval Base at Olongapo City, Zambales, a "suggestion" to respondent Judge. The Solicitor General embodied the "suggestion" in a Manifestation and Memorandum as amicus curiae.
In the case at bench, the Department of Foreign Affairs, through the Office of Legal Affairs moved with this Court to be allowed to intervene on the side of petitioner. The Court allowed the said Department to file its memorandum in support of petitioner's claim of sovereign immunity.
In some cases, the defense of sovereign immunity was submitted directly to the local courts by the respondents through their private counsels (Raquiza v. Bradford, 75 Phil. 50 [1945]; Miquiabas v. Philippine-Ryukyus Command, 80 Phil. 262 [1948]; United States of America v. Guinto, 182 SCRA 644 [1990] and companion cases). In cases where the foreign states bypass the Foreign Office, the courts can inquire into the facts and make their own determination as to the nature of the acts and transactions involved.