a) European leaders provide large amounts of military aid to Ukraine so that they can turn the Russian occupation there into an endless bloodbath. This occupies Russian attention for the next 30 years and diverts Russia from invading and destroying the EU.Unfortunately I doubt that European leaders will do option "a".
b) Massively ramp up European military spending and move large armored forces up to Russia's doorstep in order to repel their invasion (and thereby deterring it).Unfortunately if they aren't going to do "a" then they certainly won't do "b".
c) Wait until Russia invades the EU and then fight a genocidal nuclear war with Russia.Unfortunately if they aren't going to do "a" or "b" then the odds of them doing "c" are nil.
d) Sit there and do nothing as Russia invades and destroys the EU. France and England will survive, but only as individual nations, and only because they have their own nuclear arsenals. Spain and Portugal will survive as an individual nations only because France sits between them and Russia. Switzerland might survive due to their geographic defenses, but might not if Russia decides to use nuclear weapons to coerce their surrender.And it looks like it's going to be "d". It'll be a less friendly world for the US after the EU is destroyed. We may have to go back to our old Cold War policy of supporting evil dictators in order to protect ourselves from the even greater evil of Russian domination.