> Russia can't even take the country next door.

Neither could the Weimar Republic.

> With a bunch of rusty nukes that almost certainly no longer work, and that would get their (relatively few) major population centers vaporized by nukes that almost certainly work just fine?

Even 1% of Russia's arsenal being functional would be a problem, and I can imagine someone taking a gamble on "will Germany/France/England want to take a nuke to the face over the Baltics?"

Pretty much this. If a single nuke detonates over say Leipzig, it would be the most deadly single attack on European soil in history.