When the news of Tom Green’s pending promotion to major general trickled down through the ranks on Feb. 18, 1864, the men of the Texas Cavalry Brigade gave their popular leader three rousing, heartfelt cheers.

Born in Virginia and raised in Tennessee, Green had every reason to stick close to home. But he could not sit idly by while fellow Americans took on a tyrant in Mexico.

Standing an inch or two above six feet and weighing a rock-hard 200 pounds, his size probably caused Gen. Sam Housto...