I had to agree that in some stories is exaggerated the idea of a future evil empire which sacrifices masses of slaves in mines and quarries, just to show how evil they are.
This is especially problematic if one takes in to account that such economy should be already in postindustrial phase, physical work should be mostly automated and using slaves for doing McJobs doesn't look so seriously (even if that actually was economically reasonable).
Setting so far:
Side "A" - Oligarchy, at least pretends being benevolent, high HDI (let's say good compulsory education, while unhealthy lifestyle is misdemeanor), high taxes, developed industrial base, rather peaceful, thanks to high level of surveillance of its population can keep crime very low, provides bread and circuses of fine quality. Technologically beats contemporary first world countries by roughly a decade.
Side "B" - Tribal theocracy with dream of world conquest, lags terribly in tech, lacks serious industrial base, thus prefers infantry but can mobilize gigantic, fanatic armies and continue fight even after absorbing huge damage.
Side A was so peaceful that merely provided huge amount of arms and economic aid for a proxy war against B. As retaliation A was attacked by B with aim of forcing A to leave conflict or even pay contribution. A mobilized forces, prevailed thanks to superior firepower and took the invading forces as POWs.
So what would be reasonable to do with those POWs? Where to employ them?
There is no need to follow anything like Geneva Convention, though awful and pointless mistreatment of such POWs would be disliked by public opinion. (minor and justifiable is all right)
There are varied levels of loyalty among POWs (some were simply drafted and are willing to change sides inexpensively, some are fanatically loyal and willing to hide their grudge well to sacrifice their life in the right moment, some are fanatically loyal and violent all the time).
The caught POWs practically don't have any useful skills, but can be taught them.
Public opinion in A vague expectation: presumably some kind of resocialization, presumably somewhere far (classical: NIMBY), presumably in decade they can be nice but a bit odd neighbours, somewhat anxious about possible crime increase.
Elites expectation in A: whatever, but do that cheaply (at best profitably) and don't endanger social stability.