Diplomatic efforts to end the largely deadlocked war in Ukraine have focused on Western guarantees of Ukraine’s future security. But the Kremlin says it must obtain its own “security guarantees” before laying down arms. What President Vladimir V. Putin of Russia says is protection, however, would drastically limit Ukraine’s sovereignty, leaving it vulnerable to a renewed Russian attack and, many of its supporters believe, effectively turning it into a client state of Moscow. That tension over “security guarantees” — and the different ways in which the term is interpreted by the Kremlin and by the West — underlines the fundamental...