Palo Alto Battlefield National Historic Site

Nations in Conflict: A Fight Over The Republic of Texas

"The jurisdiction of the United States…has passed the capes of Florida and been peacefully extended to the Del Norte." That’s how U.S. President James K. Polk, in his December 1845 message to Congress, characterized the recent annexation of Texas by the United States, with a boundary along the Rio Grande (or Rio Bravo del Norte). He had campaigned for the Presidency with a pledge to extend the United States to the Pacific Ocean, and the addition of the Republic of Texas as the 28^th state represented a major step toward that goal.

President James K. Polk

Mexico, however, challenged annexation. Although Texas had severed ties with Mexico in 1836, many Mexican leaders refused to recognize its independence, and they denounced the U.S. move as an attack on Mexico. Even those Mexicans who had accepted the loss of Texas bristled at the claim that the Rio Grande formed the boundary of the new state. Mexico had always mapped Texas as a much smaller region – bounded in part by the Nueces River. Polk’s call for a Rio Grande border placed a huge expanse of territory in dispute.

James K. Polk attempted to force a settlement of the boundary issues. In the summer of 1845, he sent an envoy to Mexico City to negotiate an agreement. He also ordered Gen. Zachary Taylor to lead a 4,000-man army to Corpus Christi, on the Nueces River. Officially, Taylor’s army had moved to defend Texas against threatened Mexican attacks. But the show of force also was designed to convince Mexican leaders to accept the loss of Texas, agree to the Rio Grande boundary, and, perhaps, even sell the New Mexico and California territories.

The tactic failed. In Mexico, hard-line Gen. Mariano Paredes y Arrillaga marched to the capital, seized control of the government, and announced that he would discuss nothing but the return of Texas. Clear battle lines had been drawn.

General Zachary Taylor

Polk continued to exert pressure by ordering General Zachary Taylor onward to claim the Rio Grande. In March 1846 Taylor led his troops outh, occupied the bank of the river across from the Mexican city of Matamoros and began construction of Fort Texas, and earthen fieldwork that would serve as his base of operations. General Paredes showed equal resolve. He expelled the U.S. envoy from Mexico, sent thousands of troops to Matamoros, and appointed General Marano Arista to command this Army of the North.

It was Polk who finally hesitated. Many political foes at home challenged his claims to the Rio Grande boundary, and Polk realized that by moving troops to the other side of the Rio Grande or initiating conflict, he risked turning the American public against him. Instead, he ordered Taylor to wait for Mexican forces to cross the Rio Grande, which Polk could portray as an attack on American territory.

He didn’t have long to wait. Arista believed that Taylor’s army had marched hundreds of miles into Mexican territory and, in late April, ordered troops across the Rio Grande to confront the Americans. Taylor heard rumors of this Mexican movement and sent a 63-man scout party, headed by Capt. Seth Thornton, to investigate. Some 28 miles west of Fort Texas, at a tiny settlement called Rancho de Carricitos, Thornton’s party suddenly found itself surrounded by 1,600 Mexican soldiers under the command of Gen. Anastasio Torrejon. In a brief skirmish, 11 U.S. soldiers were killed and most of the rest taken captive.

Thornton Skirmish: US Mexican Battle

When President Polk learned of the "Thornton skirmish," on May 10, 1846, he announced that Mexico had "shed American blood upon…American soil" and demanded action. Although some legislators referred to the coming conflict as "the murder of Mexicans upon their own soil," on May 13, 1846, Congress responded with a vote to declare the peaceful acquisition of the Rio Grande, the United States and Mexico entered a two-year conflict to decide the matter.

U.S. President James K. Polk relied on a war with Mexico to settle territorial differences and add a vast area to the United States. His tactics, however, raised questions about national honor, the extension of slavery into newly acquired lands, and the role of the military in the United States.

Mexican President Gen. Mariano Paredes y Arrillaga seized control of the Mexican government to take a strong stand against U.S. territorial claims. By marching his troops to the Mexican capital, however, he ultimately weakened Mexican defenses along the Rio Grande.The Mexican city of Matamoros found itself at the center of conflict along the Rio Grande. Positioned on the Rio Grande near the Gulf of Mexico, it became a target for a U.S. army that wanted to "show the flag" along the river while remaining close to naval supply lines. The city also became a staging area for Mexican troops marching to the Rio Grande. Following the battles of Palo Alto and Resaca de la Palma, Mexican troops abandoned Matamoros. U.S. troops occupied the city on May 18, 1846. For the next two years Matamoros would serve as a base for U.S. operations in Mexico.