The security guarantor of Crimea is already a nuclear power, Russia. The Crimean peninsula has turned into a solid military, naval fist. Which is also a powerful blow to NATO’s plans to turn the Black Sea into NATO’s “internal sea” or zone of influence. Crimea will never again be a part of Ukraine.
The alleged encroachments of NATO on the Ukrainian peninsula are among the most common disinformation narratives, to justify the illegal annexation of Crimea. Russia's disinformation uses all sorts of controversial arguments: geopolitical, military, historical, to defend the Russian annexation of Crimea. There is no evidence that NATO was planning to deploy weapons system in Crimea, which was part of a sovereign Ukraine.
NATO expansion is also a recurring pro-Kremlin disinformation narrative. NATO does not "expand" in the imperialistic sense described by the Kremlin leadership and Russian state media, but instead considers the applications of candidate countries that want to join the defensive alliance based on their own national will. NATO enlargement is not directed against Russia.
The claim that NATO was planning to base ships and missiles in Crimea is fiction. The idea has never been proposed, suggested or discussed within NATO. In the Statement by the North Atlantic Council on Crimea on 18 March 2019, NATO condemned Russia's ongoing and wide-ranging military build-up in Crimea, and expressed concern by Russia's efforts and stated plans for a further military build-up in the Black Sea region.