I've mentioned Diane Purkiss's The English Civil War: A People's History before. I remember being struck by the way she described massacres of defeated soldiers and ordinary civilians by both sides and how propagandists from each side would write self-righteous, self-pitying dirges about the evils of their enemy.
War tends to do that.
You shouldn't buy Gordon Corrigan's The Second World War: A Military History, because 1) It's more a history of the British army in that war, with a disproportionate focus on the fighting in Burma, and 2) He makes bitchy comments and sweeping claims without following them up. He says the failure of Operation Market Garden was the fault of one of Bernard Montgomery's toadies, but he never mentioned the man once in his section about it the battle and he never explains his statement subsequently. He says that some people criticized Stalin for moving his armies westward into Poland without having built defences for them if they were attacked, but blithely comments it wouldn't have mattered one way or the other. With no explanation. 3) He's a disagreeable militarist, sexist boor, who says that soldiers should have the fear of being shot for cowardice, and says that a lot of the poor London women who were sheltered in rural communities were dirty sluts.
But his candid writing was helpful in at least one part: Writing about the rape of Nanking, Corrigan states:
All armies do go off the rails from time to time. Soldiers are aggressive and trained to be so and are a lot less squeamish about blood and slaughter than their civilian counterparts. During the Peninsular War the British Army frequently and regularly misbehaved in captured cities, but this rarely lasted more than a couple of days before exhaustion and the provost restored order. Rape is not uncommon after a period of intense fear and danger. In Nanking however what happened was far worse than a temporary loss of control.
So, what I take from that is that everyone is capable of committing atrocities and many often do. In a battle between the armies of a torturing dictatorship and jihaadist mercenaries, there's no doubt that they have. It's also true that both sides will condemn the outrages of their enemy and embellish things if need be.
These are reasons for wars to be avoided by the way.
If you want to know what I think about Syria's agonies, you can read this.