I suppose the invaded country needs to be a member of NATO at the time of invasion?

@htee Can you please clarify?

@Primer what happens if e.g. Finland leaves NATO in 2027 and then Russia invades it in 2029?

The question is interesting, but how will it be resolved if Russia attack NATO in some sort of weird way and things get confusing?

It might be unofficial attack, mistake or "mistake", provocation or both sides will claim the other side has started it.

@PatrykDunski the word "invade" suggests soldiers across a border using force to stay there.

@mpartel that is all right, but what do you count as russian soldiers?

Does it count if Russia uses wagner's group or russian separaratis? Just official russian army?

Usually Russia starts with unofficial troops first.