Maybe it was the sound of the wind cutting through the wire. Perhaps he caught a small vibration with his keen eyes. Or it could have been a slight difference in the air's smell.

Whatever it was, when Sarge noticed that his Marine Corps handler, Fred Dorr, was creeping down the wrong path in the Vietnam jungle, the German shepherd did something he'd never done out in the field: He looked at Dorr and barked, before taking a seat.