The Barbarians by Algis Budrys - History was repeating itself; there were moats and nobles in Pennsylvania and vassals in Manhattan and the barbarian hordes were overrunning the land.
It was just as he saw The Barbarian's squat black tankette lurch hurriedly into a nest of boulders that young Giulion Geoffrey realized he had been betrayed. With the muzzle of his own cannon still hot from the shell that had jammed The Barbarian's turret, he had yanked the starboard track lever to wheel into position for the finishing shot. All around him, the remnants of The Barbarian's invading army were being cut to flaming ribbons by the armored vehicles of the Seaboard League. The night was shot through by billows of cannon fire, and the din of laboring engines, guns, and rent metal was a cacophonic climax to the Seaboard League's first decisive victory over the inland invaders. Young Geoffrey could justifiably feel that he would cap that climax by personally accounting for the greatest of the inland barbarians; the barbarian general himself. He trained his sights on the scarlet bearpaw painted on the skewed turret's flank, and laid his hand on the firing lever.
Out of the corner of his eye, he caught a glimpse of another tankette rushing up on his port side. He glanced at it, saw its graceful handcrafting, and knew it for one of the League's own. He could even see the insigne; the mailed heel trampling a stand of wheat; Harolde Dugald, of the neighboring fief. Geoffrey was on coldly polite terms with Dugald—he had no use for the other man's way of treating his serfs—and now he felt a prickle of indignant rage at this attempt to usurp a share of his glory. He saw Dugald's turret begin to traverse, and hastily tried to get the finishing shot into The Barbarian's tankette before the other Leaguesman could fire. But Dugald was not aiming for The Barbarian. First he had to eliminate Geoffrey from the scene entirely.