Both the Third and Fourth Restatements of the Law, The Foreign Relations Law of the United States, were cited by the United States Court of Appeals, District of Columbia Circuit in Lusik Usoyan v. Republic of Turkey.

From the decision:

We see two issues that need to be resolved.  First, Turkey is a foreign power and—as Turkey itself concedes—its agents do not have the authority to perform law enforcement functions inside the United States. See Restatement (Fourth) of the Foreign Relations Law of the United States § 432(b) (Am. L. Inst. 2018) (“[A] state may not exercise jurisdiction to enforce in the territory of another state.”). Accordingly, if we are to find that the Turkish security detail was exercising its discretion in taking its challenged actions, we must identify the source of that discretion. Second, whatever the source of Turkey’s discretion, the plaintiffs allege that Turkey exceeded that discretion by violating various laws of Washington, D.C. We must also determine, then, whether these alleged violations take Turkey’s conduct outside the ambit of the discretionary function exception.

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We invited the United States to provide its views “on the source and scope of any discretion afforded to foreign security personnel with respect to taking physical actions against domestic civilians on public property.” In its brief, the United States declares that no source of positive law explicitly grants Turkey the authority to use physical force in the protection of diplomats on U.S. soil. Instead, the United States locates Turkey’s right in customary international law:

The principle that sending states are authorized to protect diplomats and officials traveling abroad has not been codified in a treaty, as has the obligation of receiving states to protect foreign diplomatic and consular personnel, but that does not reflect any uncertainty about whether the authority exists. To the contrary, this principle is widely accepted in international practice and reflects the fact that nations have inherent authority to protect their diplomats and senior officials outside their borders, subject to the authorization of the receiving state.

Although the United States does not use the phrase “customary international law,” that is the clear implication of its reference to international practice and the “inherent authority” of nations. Customary international law, after all, is simply the “general and consistent practice of states followed by them from a sense of legal obligation.” Restatement (Third) of the Foreign Relations Law of the United States § 102(2) (Am. L. Inst. 1987).