The Thornton Affair, When Mexico Invaded the US
In 1835, after about a decade of mounting tensions between the Mexican government and the steadily growing American immigrant population in its northern state of Texas, the immigrants rose up in rebellion. The following year, after defeating an army sent to stamp out the rebellion and bring Texas back under the Mexican government’s control, the immigrants declared independence and proclaimed the Republic of Texas.
Most Texans, a majority of whom were American immigrants, wanted to join the US. However, both major political parties at the time were leery about adding a vast slave holding state into the volatile and often toxic political climate surrounding the slavery question. They also wanted to avoid a war with Mexico, whose government did not acknowledge Texas’ independence, and viewed it as a rebellion province temporarily outside the Mexican government’s control.
However, in 1843, American president John Tyler, on the outs with both parties, decided to annex Mexico in order to build a popular base of support for a reelection bid. Tyler did not manage to get reelected, but he did manage to get Texas added to the US. After a 9 year run as an independent country, the Republic of Texas was annexed to the United States, and admitted into the Union as the 28th state on December 29th, 1845.
As a result, tensions mounted between the US and Mexico, who officially still saw Texas as part of its territory. Matters were further exacerbated by disagreements about just where Texas’ border with Mexico lay. Mexico claimed that the border lay at the Nueces River, while the US claimed that the border lay at the Rio Grande, hundreds of miles to the south. The disputed region between those two rivers thus became a flashpoint.
In March of 1846, the US Army began taking up positions north of the Rio Grande, in the territory claimed by Mexico. The Mexican government responded by sending an army to clear what it considered its territory of foreign forces, and on April 24th, the Mexican army began crossing the Rio Grande. Upon receiving the news, the US commander sent two dragoon companies of about 80 men, under the command of a Captain Seth B. Thornton, to investigate.
On April 25th, 1846, Thornton ran into a Mexican force of about 1600 men, and his command was largely wiped out. Of his 80 men, 11 were killed, 6 were wounded, and 49 were captured. The incident, which came to be known as “The Thornton Affair”, became a casus belli. As newly inaugurated president James Polk put it when asking a joint session of Congress for a declaration of war: “Mexico has passed the boundary of the United States, has invaded our territory and shed American blood upon the American soil“. The US won the ensuing American-Mexican War, in which Mexico lost all of its northern provinces. In the 1848 Treaty of Guadalupe Hidalgo, which ended the war, Mexico recognized the Rio Grande as the US-Mexican border, making the Thornton skirmish a foreign attack on US soil.