Forgetting that he was only a brute, he posited that this was no more than a brute with which he strove to play in the genial comradely way that the Skipper played.
Red war was inevitable--not first on Jerry's part, but on Borckman's part.

Borckman felt the abysmal urgings of the beast, as a beast, to prove himself master of this four-legged beast.

Jerry felt his jowl and jaw clutched still more harshly and hardly, and, with increase of harshness and hardness, he was flung farther down the deck, which, on account of its growing slant due to heavier gusts of wind, had become a steep and slippery hill.
He came back, clawing frantically up the slope that gave him little footing; and he came back, no longer with poorly attempted simulation of ferocity, but impelled by the first flickerings of real ferocity.

He did not know this.

If he thought at all, he was under the impression that he was playing the game as he had played it with Skipper.