Fake Vilnius security guarantees are worse than useless. It will keep Ukraine a target. The war itself is testament to that, as it seems quite unlikely this conflict would have occurred absent Russia's belief that Ukraine was on its way to security integration with NATO, violating its "brightest of redlines," as CIA Director Bill Burns put it while U.S. Ambassador to Ukraine.
Of course, saying this does not excuse Russia's aggression, but it does make its repetition predictable - if the West is to also repeat its policy of keeping the door open to fighting for Ukraine someday. Russia might not necessarily believe such proffered protection will be real, but it may still view it as threatening to its perceived interests, particularly if NATO troops or infrastructure were to be deployed to Ukrainian territory. Moreover, holding out the prospect of Ukraine joining NATO, or getting future security guarantees of another sort, would encourage Russia to keep the war going. In fact, providing security guarantees now would be even worse, forcing the U.S. to either ignore its commitment and undermine other alliances, or fight for Ukraine and spark an immediate nuclear crisis.
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