Kill the royal couples, no problem. There is an argument to be made that those that start wars should be sentenced to death for doing so. Particularly frivolous ones of aggression.
If the civilian populous has access to arms and armories they can make it a worthless proposition to invade them. Its not like the world is entirely composed of large countries with strong militaries already. An insurgency can last decades under fire from a superior force, an organized military won't. So if you can't match the organized military might of potential enemies, its probably mostly a waste to try.
In front of a blood-stained chessboard littered with mutilated chess pieces finely dine two royal couples - black and white - cheering their endgame.