Really, in the hypothetical case, any sovereign state can declare war on any other. And it need no longer be ordinance fired and boots on ground....I'd rather see a nation deploy an EMP against the Vatican's communications array, freeze any of their financial assets within the offended nation's area of control, and counter sue them in court.
Technically, the U.N. could sponsor sanctions, right? Or is Vatican City even a U.N. member state?