The main reason for Article 18 being one of the most opaque provisions of the Vienna Convention is that it establishes a relatively vague 'interim obligation' for States to refrain from acts which would defeat the object and purpose of a treaty between its signature and ratification. Although the existence of such an interim obligation has been recognized by States and in various international legal regimes, it remains problematic since Article 18 neither defines nor determines its own contours and when and under which conditions it is being breached. It goes without saying that the legal consequences of a possible breach of this provision are left equally unclear. It remains uncertain how the interim obligation of Article 18 fits into the general international law of treaties; what its legal nature and temporal scope is; which role the principle of good faith plays as a possibly underlying principle of this provision; and how we should understand the object and purpose of a treaty and how it can be defeated. Furthermore, its apparent focus seems to be on bilateral rather than multilateral treaties, but this exclusive application of this interim obligation to bilateral treaties would contravene both the expressed and implied intent of the drafters. Therefore, this article also discusses how Article 18 fits within the normative system of international law and law-making treaties, such as human rights treaties.
Gragl, P., & Fitzmaurice, M. (2019). The legal character of article 18 of the vienna convention on the law of treaties. International and Comparative Law Quarterly, 68(3), 699–717. https://doi.org/10.1017/S0020589319000253