Although it is currently fashionable for many of the current crop of philosophers to disdain the practice of slavery, it persists in all the far-flung places of the world, from the nomads of the southern deserts to the Notable exceptions to this include the Orcs, who loathe the very idea of slavery, the Dwarves, who have little use for it (but are generally not opposed for any moralistic reason), and the Norvu, who only rarely keep war slaves.
It is here that the distinction should be made between born slaves and war slaves. Born slaves are born into their life of pure servitude, or are otherwise reborn into it as women or children captured in war. They only rarely gain emancipation, though some city-states do stipulate that the third generation of slave should be considered freeborn.
War slaves are a different, higher class of slave. They are warriors captured in battle, but are treated with respect as befits their station. They may be enticed to join their captives in war, so long as it is not against their own people. This period of slavery can persist until the captured warrior is paroled, or at times for a period of ten years. Many kingdoms also reward valorous war slaves by granted their emancipation after a particularly noble act, though just as many see war slavery as a lifetime contract. Indeed, in the Hegemony of Aerris, war slaves are set aside to be used as warrior in their elaborate gladiatorial games.
Of course, Your Majesty is well aware that Aquona has a relatively minor number of born slaves, but does have a goodly amount of war slaves, many of whom have fought well in your service and have been given their freedom once more. Such benevolence is a fine mark of wise kingship.