No country is entitled to sovereignty. Sovereignty isn't given, it's taken by force. What Russia did by invading Ukraine was terrible, but the Ukrainians essentially let their own country disintegrate and failed to maintain even the slightest deterrent. Coming to their rescue now would create a huge moral hazard: other countries in the region might think they no longer have to defend themselves against Russian aggression because if they get into trouble them NATO will ride in to save them.