Captain Seth Thornton and his U.S. Dragoons made their way through chaparral thickets of what was then a much contested South Texas.

Thornton and his men had been dispatched out of Fort Texas by General Zachary Taylor about 30 miles due west from where Brownsville now sits. The U.S. troops would eventually get to a spot about eight miles southeast from today’s Mercedes to what was known as Rancho de Carricitos. It was April 25, 1846, and there were reports of Mexican troops crossing the Rio Grande into territory two countries claimed.

The rancho was little more than a clearing with a cluster of homes in it. This seemingly quiet spot would be the place where a war between the United States and Mexico would be triggered. Neither nation would ever be the same again.

When Thornton and his men left Fort Texas, they did so in the midst of a tense standoff between Mexico and the United States. Both countries laid claim to the strip of Texas south of the Nueces River. U.S. President James Polk had ordered Taylor and hundreds of U.S. troops to march south from Corpus Christi toward the Rio Grande.

Mexican troops were waiting in Matamoros across from Fort Texas. It was against this backdrop that Thornton and his troops walked into at a spot not far from today’s Pfc. Pedro Martinez Rd./FM 491, which leads into Mercedes. There were indeed Mexican troops north of the Rio Grande – and they were at the rancho with the clearing. Intense gunfire ensued when the opposing armies met up at Rancho de Carricitos.

The Mexican troops were led by Gen. Anatasio Torrejon and his forces greatly outnumbered those of the opposing U.S. forces. Thornton and his men would be surrounded. The American captain was captured and taken prisoner. Eleven of his soldiers lay dead on a battlefield. It gave President Polk the rationale he needed to declare war on Mexico.

“American blood has been spilled upon the American territory,’’ Polk declared to Congress on May 10, 1846.

What came to be known as the Thornton Skirmish led Congress to declare war against Mexico on May 13, 1846. A young congressman from Illinois rose in a dissenting voice to challenge Polk’s claim.

“Show me the spot where American blood was shed,’’ U.S. Rep. Abraham Lincoln said, questioning the constitutionality of the war.

It was not enough to keep Polk from the war he desired that would lead to the United States adding an immense amount of land to its union, including Texas and California. The exact location just southeast of today’s Mercedes where Polk said American blood was shed remains a mystery. The Palo Alto Battlefield Park is performing surveys and research to try to pinpoint the exact spot.

For now, a cannon and two plaques commemorate the famous battle that triggered war that so profoundly changed the histories of two neighboring countries. Cornfields sway in the Rio Grande Valley wind along the Military Highway behind the three markers honoring a profound event in U.S. history.

“The spot were American blood was shed on American soil,’’ one plaque on the Military Highway states.

Those words as said by a U.S. president nearly 175 years ago point to the historical importance of Mercedes and the Valley.

• Ricardo D. Cavazos