His teeth came together with a snap.
"It's the last time," he said aloud--meaning that Mrs.Fortescue would have to submit to his judgment in horses and let Birdseye alone.
What happened next turned the Colonel's resolution to adamant.

A trooper was leading Pretty Maid away and another trooper was about to do the same for Birdseye when the black mare suddenly threw her head down and her heels up.

Mrs.Fortescue kept her seat, while the mare, backing, and kicking as she backed, knocked over a couple of the passing color guard, and only by adroitness the color sergeant saved the flag from being dropped to the ground.

Meanwhile, the two troopers, falling backward, collided with the chaplain, a small, meek man, as brave as a lion, who stopped to look and was ignominiously bowled over.

Sergeant McGillicuddy, just coming out of the office entrance, made a dash forward and grabbed Birdseye by the bridle.