But the other states don't matter! If Libertarian country A is in trouble (from an aggressor, not of its own doing) and free citizens from Libertarian country B want to get involved, they are free to do so. It's none of B's government's business! It would not even occur to a Libertarian that he or she might require the government's permission.

Tho' given that the Libertarian value is long-term freedom, it's unlikely for a Libertarian nation (not the same thing as s state) to be caught napping. A Libertarian country is likely to have a well-trained and well-armed militia, but not much equipment or experience or inclination for expeditionary warfare.