After the Mexican Congress ratified the 1848 Treaty of Guadalupe Hidalgo, the Rio Grande (Rio Bravo) was the legal boundary between Texas and Mexico. Under the treaty, the United States was obligated to prevent raids by "hostile tribes" in Mexico whose northern frontier had been ravaged by the raids. This obligation was accepted despite the absence of a wagon road between San Antonio and El Paso or any U.S. Army forts with soldiers stationed along the border. In fact, no Americans, including Texans who claimed the lands, knew where the border or tribal crossings were located. This is the...