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 story of the 1848 Hays Expedition, the first U.S. effort to search for a wagon road route along the new border to Chihuahua and El Paso. The original intent was to establish a trade route to Chihuahua but the Expedition’s efforts to explore the new lands proved to be far more difficult. Besides crossing the most rugged terrain in Texas with almost no water sources and starving from lack of food, the Expedition survived the first American exploration of the Texas-Mexico border and provided critical information that led to the settlement of far West Texas and a new route from the Gulf of Mexico to the Pacific Ocean.