France's Cour de cassation has quashed a 2023 Paris Court of Appeal judgment, ruling that courts reviewing an arbitral tribunal's jurisdiction must give weight to a later interpretative agreement between States even where the underlying treaty is silent on the disputed point. The First Civil Chamber delivered its decision on 6 May 2026 (nº 24-10.445) and published it in the Bulletin.
The dispute traces back to an investor born a Vietnamese national who acquired United States nationality by naturalisation in July 2014. Through two Texas companies, U.S. Global Institute Inc. and Angels Company Inc., she invested in a power-plant project in the Socialist Republic of Vietnam by taking shareholdings in Tan Tao Energy Corporation. After Vietnam cancelled the project in March 2016, she alleged governmental measures amounting to expropriation.
In September 2019 the investor filed a request for arbitration under the 1976 UNCITRAL Arbitration Rules, invoking the 2000 Agreement between the United States and Vietnam on Trade Relations (the BIT). The arbitral tribunal issued a partial award on 8 December 2021 in which it upheld its jurisdiction. Vietnam brought an action for setting aside that award before the French courts, but the Paris Court of Appeal dismissed the application on 12 September 2023. Vietnam then appealed to the Cour de cassation.
The interpretative question
Vietnam's argument centred on treaty interpretation under Article 31 of the Vienna Convention on the Law of Treaties of 23 May 1969, which it framed as a rule of customary international law binding on the French judge. Article 31(3)(a) requires that, together with the context, any subsequent agreement between the parties regarding the interpretation of a treaty or the application of its provisions be taken into account.
The centre of the controversy was a diplomatic note issued on 4 April 2023 and addressed to Vietnam by the United States Embassy. The Paris Court of Appeal had treated the note as no more than an opinion of the Embassy's Economic Service, whose authority it considered unestablished, and therefore held it ineffective for determining the contracting States' intention. It reasoned that the BIT contained no provision on investors holding dual American and Vietnamese nationality, its only requirement being that a natural person be a national of a Party, and that nothing justified reading a distinction into a text that drew none.
What the Cour de cassation held
The Cour de cassation confirmed that, under Article 1520, 1° of the Code of Civil Procedure, an action for setting aside lies only where the arbitral tribunal wrongly declared itself to have or to lack jurisdiction. Examining the interpretative issue, it reached the opposite conclusion to the Court of Appeal.
The Court held that the 4 April 2023 diplomatic note constituted an agreement reached between the States Parties on the interpretation of the BIT, addressing precisely the situation on which the treaty was silent: that of an investor holding dual American and Vietnamese nationality. On that reading, dual nationals were excluded from the protection the BIT affords.
The Court concluded that a court breaches the customary rules reflected in Article 31(3)(a) where it refuses to consider diplomatic exchanges between the States parties establishing a shared interpretation, particularly where the treaty neither expressly excludes dual-national investors nor draws any distinction. It accordingly set aside the 12 September 2023 judgment in all its provisions.
The ruling carries significance for arbitration practice on several fronts: treaty silence on dual nationality did not close the inquiry; subsequent State practice and interpretative agreement proved decisive; and French courts confirmed that, when reviewing jurisdiction, they may closely examine whether an arbitral tribunal correctly applied international law rules on treaty interpretation.