But ISIS isn't an army in the historical sense as it doesn't really have a way to deal with POWs who would have to be housed, fed, guarded, etc. It's much easier to kill them.
True. But “armies” that aren’t really armies historically have a much harder time holding onto gains than making them in the first place.
And your point doesn’t change anything about the negatives of murdering prisoners.
That’s why I personally suspect ISIS will retreat soon, possibly even more rapidly than they advanced.
It seems obvious to me that the answer in Iraq is to split the country into three. But there is no “good” answer to the situation, as even that would entail massive ethnic cleansing and bloodletting.
It seems to me the collapse of the existing order in the Middle East is due more to internal tensions than to anything the USA has done or not done. But Americans, right and left, like to believe that “If only we’d done (or not done) X, Y or Z, everything would have been OK.”
The real answer for us, though not for the Middle East and the rest of the world, is to make ourselves energy-independent and let the ME pound sand.