While not required in a legal sense they certainly might do so anyway. Ukraine is as entitled to it's sovereignty as any other nation. Just letting the Russians steamroll Ukraine is equally irresponsible. Germany, France, etc are going to (rightfully) have a very, very hard time tolerating (i.e. not coming to the aid of Ukraine) that kind of belligerent behavior in Europe.
Ukraine is not part of Europe as far as "Germany, France, etc" are concerned. These countries are concerned about the interests of the EU. Also, most of the EU has strong trade relationships with Russia (e.g. via Gazprom).
As long as Russia does not pose a direct threat to the EU, there's very little incentive to jeopardize trade relationships to uphold ideals in foreign countries.
I was specifically addressing the Budapest Memorandum. It often gets brought up under the mistaken belief that it requires the signatories to defend Ukraine, and thus that the other signatories are in violation for not defending Ukraine against Russia.
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.
