In 1846-1847, a ragtag army of 800 American volunteers marched 3,500 miles across deserts and mountains, through Indian territory and into Mexico. There they handed the Mexican army one of its most demoralizing defeats and helped the United States win its first foreign war. Their leader Colonel Alexander Doniphan, also a volunteer, was a "natural soldier" of towering stature who became a national hero in the wake of his wartime exploits. Doniphan...