Why can't Europe defend itself?

[Vuk]

because, militarily, it is a wreck.

never again will russia have the ability to pose an existential (conventional) threat to europe, and there would be no point nuking it either because it has not the resources to consolidate its victory, presupposing it survived a second strike itself.

russia can bully its little neighbours, but that is it.

lol, I think that you are underestimating them, and the fact that they would most likely have China as an ally. They do not even have to occupy most of the countries as long as they get they own regimes in power, and use the resources of the country to suppress itself.