From Brzak v. United Nations, 597 F.3d 107 (2d Cir. 2010):

[T]he United States has ratified the CPIUN [the Convention on Privileges and Immunities of the United Nations, Feb. 13, 1946]***

The parties do not dispute that the CPIUN is binding on the United States as a matter of international law. However, they disagree about whether American courts must recognize the immunity it adopts in domestic litigation. Cf. Medellin v. Texas, 552 U.S. 491, 504, 506, 128 S. Ct. 1346, 170 L. Ed. 2d 190 (2008) (acknowledging that an International Court of Justice opinion is binding on the United States as a matter of international law, while holding that the same opinion lacks domestic legal effect).

Brzak and Ishak contend that the CPIUN should not be enforced by American courts because it is not self-executing, and consequently cannot be enforced absent additional legislation which was never passed. *** Whether a treaty is self-executing depends on whether "the treaty contains stipulations which ... require no legislation to make them operative;" if so, "they have the force and effect of a legislative enactment." ***

In determining whether a treaty is self-executing, we look to the text, the negotiation and drafting history, and the postratification understanding of the signatory nations. *** Additionally, the executive branch's interpretation of a treaty "is entitled to great weight." ***

CPIUN Section 34 states "[i]t is understood that, when an instrument of accession is deposited on behalf of any Member, the Member will be in a position under its own law to give effect to the terms of this convention." When the United States acceded to the CPIUN in 1970 (by the President's ratification, with the advice and consent of the Senate), it was affirming that it was "in a position under its own law to give effect" to the CPIUN's terms at that time. This means that the treaty became effective at ratification, and therefore, is self-executing. "[T]he label 'self-executing' usually is applied to any treaty that according to its terms takes effect upon ratification." Mora v. New York, 524 F.3d 183, 193 n.16 (2d Cir. 2008) (quoting United States v. Li, 206 F.3d 56, 67 (1st Cir. 2000) (en banc) (Selya & Boudin, JJ., concurring)).

The ratification history of the CPIUN reinforces this conclusion. During testimony before the Senate Foreign Relations Committee as it considered whether to recommend that the Senate ratify the CPIUN, the Legal Advisor to the State Department stated that: "It is clear from the language of the convention... that the convention is self-executing and no implementing legislation is necessary."*** The Foreign Relations Committee's report on the CPIUN also expressed the view that "the convention is self-executing and will require no implementing legislation." ***

Finally, the executive branch continues to assert that the CPIUN is self-executing. *** These views, as we have seen, are entitled to "great weight."***

Our conclusion is further confirmed by the International Organizations Immunities Act of 1945,22 U.S.C. § 288a(b) (the "IOIA"), which provides that international organizations designated by the President should receive the "same immunity from suit and every form of judicial process as is enjoyed by foreign governments." The United Nations has been so designated. *** The plaintiffs argue that designated international organizations no longer have absolute immunity in all cases, because, since that act was passed, Congress has passed the Foreign Sovereign Immunities Act, 28 U.S.C. § 1602-11("FSIA"), which strips foreign sovereigns of their immunity in certain circumstances. Plaintiffs argue that it is this narrower definition of sovereign immunity that now defines what sort of immunity the IOIA applies to international organizations. Although this argument has been rejected by at least one other Court of Appeals, see Atkinson v. Inter-American Dev. Bank, 156 F.3d 1335, 1340-42, 332 U.S. App. D.C. 307 (D.C. Cir. 1998), we need not resolve whether plaintiffs' argument is correct for at least two reasons. The first is that, whatever immunities are possessed by other international organizations, the CPIUN unequivocally grants the United Nations absolute immunity without exception. The second is that the plaintiffs have not presented any argument, either at the district level or to us, which would suggest that one of FSIA's exceptions to immunity would apply. ***

The plaintiffs also sued three former United Nations officials. The CPIUN also addresses their immunity: "The Secretary-General and all Assistant Secretaries-General shall be accorded ... the privileges and immunities ... accorded to diplomatic envoys, in accordance with international law." *** As we have determined above that the CPIUN is a self-executing treaty, this provision is binding on American courts. International law provides extensive protection for diplomatic envoys. See The Vienna Convention on Diplomatic Relations, Apr. 18, 1961, entered into force with respect to the United States Dec. 13, 1972, 23 U.S.T. 3227 (the "VCDR"). Although current diplomatic envoys enjoy absolute immunity from civil and criminal process, *** former diplomatic envoys retain immunity only "with respect to acts performed by such a person in the exercise of his functions" as a diplomatic envoy. *** As the plaintiffs have sued former United Nations officials, each of whom held a rank of Assistant Secretary-General or higher, it is this functional immunity, which the CPIUN incorporates by reference, that is relevant. The Diplomatic Relations Act of 1978, 22 U.S.C. § 254d, makes pellucid that American courts must dismiss a suit against anyone who is entitled to immunity under either the VCDR or other laws "extending diplomatic privileges and immunities." As CPIUN section 19 is such a law, the remaining question is whether the plaintiffs' allegations against the individual defendants involve acts that the defendants performed in the exercise of their United Nations functions.

When a court attempts to determine whether a defendant is seeking immunity "with respect to acts performed by such a person in the exercise of his functions," VCDR art. 39, para. 2, the court must do so without judging whether the underlying conduct actually occurred, or whether it was wrongful. ***