By that line of reasoning, I should be legally allowed to invade my unarmed neighbor's house, kick him out and claim it for myself.
The question of "legally" invading someone's house is odd anyway, because legality implies some authority having deemed it appropriate. In the case of a neighbor's house, it is the government of the country containing the house. I can't think of an entity that establishes the "legal" way of creating/invading a country.
Of course this is all messed up. There's no reason a nation shouldn't be considered a nation just because it doesn't have a military.