“They became the most effective fighting force that Mexico had, largely because they knew how to use the armaments of the U.S. Army,” said Professor Greenberg, author of “A Wicked War: Polk, Clay, Lincoln, and the 1846 U.S. Invasion of Mexico.” They had a special talent for capturing American cannons, she added, “and for using them against the Americans incredibly effectively.”

Here at the battle of La Angostura, which the Americans call the Battle of Buena Vista, the San Patricios occupied a spot near the base of the hillside, just below where the gravel mine can now be found. By most accounts, they pounded the Americans, and kept them from advancing for two days.

But it was not enough. The Mexican commander, Gen. Antonio López de Santa Anna, retreated after the second day of battle on Feb. 23, 1847. His troops outnumbered the Americans three or four to one, and many historians argue that the Mexicans were defeating the ranks of volunteers and regulars led by Gen. Zachary Taylor, whose hero status in the war ultimately led him to the White House.

General Santa Anna fled anyway, essentially giving up on the war for northern Mexico. Some historians have theorized that he left because his soldiers needed food and water or because he needed to deal with an internal rebellion back in Mexico City. But Mr. Rodríguez blames one of Mexico’s ugliest flaws: corruption.

“When you look at this closely, you have to think he made a deal,” Mr. Rodríguez said. “It encapsulates the whole problem of Mexico.”

And perhaps that helps explain the struggle to draw more attention to the battle. Mr. Rodríguez and his partner at the museum, Isidro Berrueto Alanis, say they dream of a day when these hills are a military park where Americans and Mexicans can walk together, as Southerners and Northerners tour Gettysburg, Pa.

But for many, the past still pricks the sensitivities of the present. When Absolut Vodka ran an ad in Mexico a few years ago with a map showing California, Texas and much of the West belonging to Mexico, there were threats of a boycott in the United States as many Americans insisted that the company was calling for redrawn borders or mass Mexican immigration.