There was a broad awareness of how bad war could be. Chaucer, in his description of the Temple of Mars in "The Knight's Tale," gives a vivid scene of the horrors of war some of which he may have borne witness to whilst serving Edward III in France. Here's one translation of the passage in question: http://www.librarius.com/canttran/knighttrfs.htm, which I'll quote below.

Choice passages to me, with regards to what you're asking about would be lines 1153-1165:

"And saw I Madness laughing in his rage;
Armed risings, and outcries, and fierce outrage;
The carrion in the bush, with throat wide carved;
A thousand slain, nor one by plague, nor starved.
The tyrant, with the spoils of violent theft;
The town destroyed, in ruins, nothing left.
And saw I burnt the ships that dance by phares,
The hunter strangled by the fierce wild bears;
The sow chewing the child right in the cradle;
The cook well scalded, spite of his long ladle.
Nothing was lacking of Mars' evil part:
The carter over-driven by his cart,
Under a wheel he lay low in the dust."

I would note that this was arguably the leading English poet of the fourteenth century, employed by both Edward III and Richard II. Although I would also note that Chaucer wasn't anti-war; he presents the Knight in The Canterbury Tales with admiration; it's just that he knew how bad it could become.