The Memorandum on Security Assurances in Connection with Ukraine’s Accession to the Treaty on the Non-Proliferation of Nuclear Weapons (NPT), signed in 1994 by Ukraine, and the three NPT depositary states, the United States, United Kingdom and Russian Federation, is not a ratified, legally binding treaty, but a set of high-level political commitments. Its significance is in explicitly linking Ukraine’s sovereignty and security to the NPT, and the basic bargains enshrined in it. The NPT’s depositary states gave Ukraine security assurances in exchange for Kyiv’s renunciation of the world’s third largest nuclear weapons arsenal. Moscow’s manifest violation of this deal when it invaded Ukraine in 2014 not only eroded European security. It has also undermined the rationale of the international nonproliferation regime. While not directly related to the question of the North-Atlantic Alliance’s enlargement, Russia’s disregard for the NPT’s logic has, since 2014, escalated Ukrainian critique of NATO’s insufficient engagement in Eastern Europe, during the last quarter of a century.