Under the pretext of compensating him for the theft of his horses, Mexican soldiers lured a Rio Grande rancher to their side of the river on Jan. 12, 1914.

Forty miles north of Laredo and a stone’s throw from Hidalgo, Mexico, there once was a poor imitation of a town called Palofox. Clemente Vergara lived on the bluff above the border crossing and used a small island in the shallow Rio Grande as a natural corral for his horses.

Dolores Vergara looked out her window on a January morning in 19...