Side A: Technocracy, highly industrialized, rich, hedonistic, peaceful (sort of ;) ) Side B: Theocracy, poor, ascetic, openly declares that it want to conquer all non-believers
Assets: Side A: Small, disciplined, theoretically well trained but inexperienced in real fight army, huge amount of mothballed semi autonomous combat vehicles (equivalent of ICV, aerial, even some slow submarines), maybe somewhat squishy society, but very effective state apparatus that allows them to mobilize needed resources, operating with very short supply lines. Side B: Huge, generally fanatic army, poorly equipped (practically lack of heavy weapons), some units are really battle hardened.
Soldiers of side B seriously believe that they are facing rich but weak and coward oponent. They correctly know that technically speaking army of Side A is outnumbered many times (They've seen that Side A was using women on front line, so they must be really running out of soldiers). They know that side A is even by its allies perceived as sexually depraved, so the conclusion is that any POW taken by Side A would be treated in really cruel and unusual way, thus even not specially fanatic soldiers would think about fighting to death.
From early assessment of Side A, they should be able to easily win that war. Wherever Side B sends its soldiers, it would have to have them concentrated on small territory at start and be subject to really lethal bombardment using chemical weapons and cluster bombing. What would survive that would be harassed by air forces and outmaneuvered by land forces. All really valuable targets are anyway protected by lines of mine fields, barbed wire and semi autonomous machine gun turrets.
Nevertheless, leadership of A, wonders whether they could instead of killing almost all enemies just give them a chance to surrender. Not only its perceived as more civilized but also POW could be used as unfree labour and later used in a prisoner swap (some of its allies soldiers and civilians were captured).
Is there a practical way to achieve that? (it's not an aim that has to be desperately achieved)
I was thinking in line of dropping propaganda leaflets explaining Side A rules of treating POWs (which actually are not so bad), with dropping either either Buzz or moderate amount of mustard gas, hoping that it would rather incapacitate enemy without just killing them all. And later encircle separate enemy units and force them to surrender using artillery.
Would such approach would be worth a try? Is there a better way of trying to take enemy as POWs? Or maybe there is no good way, and if they want to be pragmatic and humanitarian they should just use whatever would cause quick death?
In this setting there are de facto no convention concerning war conduct.