Nothings stopping any European force from aiding the Ukrainian forces on the ground The Poles especially have very good reason to hate Russians, but also to wish to stop Russian plans for any more advancing on other countries,
same can be said for every other country that suffered under the Soviet rule, post WW2, I think the threat of Russia using Nuclear, or chemical weapons, is more postering than actual threat, tactically, they would ensure Russia loses, in a big way, plus, there is always the chance of the fall out being blown back over their own people,
and then, there would be the response, Nato would send in its QRS units, battle groups, with full support, and Russian forces in Ukraine, would be wiped out in 48 hours, and their ability to wage war ended, A singe cruise missile hitting any UK target would see an immediate NATO response,
And if it were nuclear, that's game over, either way, there would be an overwhelming response .