Swordships of Scorpio by Alan Burt Akers

What does a man do when fate makes him the protector of the royal head of the land of his enemies? If he is Dray Prescot, Earthman on Antares, he sets aside his quest to do his duty. His duty is to reach Vallia and his princess Delia and help her claim her throne. His duty is to defend Vallia's ancient foe and place its rightful heir on its throne -- sworn to attack Vallia. So when the third force, the pirate fleets known as the swordships, come between the two contending demands, Dray sees that only by following his own personal star could the contradiction be resolved.

Swordships of Scorpio is the fourth book in the epic fifty-two book saga of Dray Prescot of Earth and of Kregen.