How do you conquer ungoverned land?

[Matty1Monopoly]

I'm playing as Germany and I've beaten France & UK so badly that their countries & Spain are now ungoverned lol.

[Ninjafroggie]

you have to be at peace with all other major powers, then hope RNG likes you and triggers an event to invade.

[SentosKarum]

What the previous poster said, although I am not sure about the peace bit. In addition it helps having a land province bordering.

(Some land provinces, like Gibraltar, do not seem to be used as a basis for a land offensive - but also not as target, either)