We need to show Atvatabar that they can't win. So we choose their biggest ship, and use our biggest gun with the biggest explosive shell.
Just one shot, and it's spectacular! We aim just below the water, so the low gravity creates an unforgettable show. One moment there is a gigantic ship, the next? a tower of water and matchwood.
As the debris falls we ask the king to surrender. But he knows that if he does his whole culture is finished. if a single American ship can destroy their entire empire then sooner or later Atvatabar will become just a mining colony of the United States.
So the king and the old timers - millions of people - are ready to sacrifice themselves. Already the remaining ships are moving toward us, trying to get close enough to fire their air guns.