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This Day in History: Thornton’s Affair sparks the Mexican-American War

On this day in 1846, Mexican and American forces clash in a disputed area close to the Rio Grande. The so-called Thornton Affair would finally push the United States into war with Mexico.

“Hostilities may be considered to have commenced,” then-Brigadier General Zachary Taylor wrote President Polk. “American blood has been spilled.”

Tension had been mounting for years. Texas won its independence from Mexico in 1836, but the two sides never really agreed to a boundary after the war. Instead, the disagreement persisted even after Texas joined the United States in 1845. Soon afterwards, Taylor and an Army of Occupation were dispatched toward the southern border.

They would establish the Rio Grande as the southern boundary of the United States.

Taylor arrived across the river from the Mexican city of Matamoros on March 26, 1846. He began building a fort (Fort Texas) near present-day Brownsville.

On April 24, Taylor received reports that Mexican forces were beginning to cross the Rio Grande. Something had to be done. He ordered Captain Seth Barton Thornton to investigate lands to the west of the fort. Thornton was to report back by noon the next day.

The Battle of Palo Alto, fought near Fort Texas soon after the Thornton Affair.

What followed is a matter of some dispute, but simple exhaustion surely affected the decisions that were made. Thornton’s men were up all night, moving slowly through new territory. They talked with locals as they went, receiving conflicting reports about Mexican activity in the area. Early on April 25, they found a plantation and went to investigate.

Mistakes were made.

Thornton failed to leave a force guarding the open gate at the front of the plantation. He’d gone in and was looking for someone who might have information on Mexican activity in the area. His men were also inside the gates, but in varying states of readiness.

Suddenly, about 1,600 Mexican soldiers appeared on the scene.

The scene that followed was chaotic. The plantation lay close to the river, so Thornton’s men were penned in, trapped by the river and by fences. Men and horses panicked. They were badly outnumbered, and the skirmish ended with 11 Americans dead, 6 wounded, and close to four dozen captured.

Armed conflict followed a few days later when Mexican forces began bombarding Fort Texas. Americans rebuffed the attack, but things had reached a tipping point.

President Polk addressed Congress, asking for a declaration of war.

“The cup of forbearance had been exhausted even before the recent information from the frontier of the Del Norte,” he told Congress. “But now, after reiterated menaces, Mexico has passed the boundary of the United States, has invaded our territory and shed American blood upon the American soil. She has proclaimed that hostilities have commenced, and that the two nations are now at war.”

Congress agreed, declaring war on Mexico on May 13, 1846.

The Mexican-American War continued for more than a year: The final military blow came in September 1847 when Americans seized the Mexican capital. A treaty was signed in February 1848.

It had been nearly twelve years since Texans first won their independence from Mexico. Now, at long last, it was decided: Mexico would finally recognize the Rio Grande as the border of Texas.

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