The Siege of Fort Texas marked the beginning of active campaigning by the armies of the United States and Mexico during the Mexican–American War. Fort Texas was constructed by order of General Zachary Taylor on the northern side of the Rio Grande opposite the Mexican town of Matamoros, at a time when the Rio Grande border with the United States was disputed by Mexico. The establishment of this fortification and subsequent Mexican response initiated the first major military engagement of the conflict.
On 3 May 1846, Mexican forces under General Mariano Arista opened fire on and besieged Fort Texas, which was garrisoned by 500 men under Major Jacob Brown. The fort had been designed as an earthen star fort with capacity for 800 men and included the 7th Infantry. In response to the siege, the main American force under General Zachary Taylor advanced from Port Isabel and engaged Arista's Mexican army at the Battle of Palo Alto on 8 May 1846. The following day, Mexican forces were routed at Resaca de la Palma, located 4 miles from Fort Texas.
The Mexican withdrawal from the siege had significant consequences for the course of the war. The Mexican force withdrew south of the Rio Grande, effectively ending the siege. Taylor subsequently occupied Matamoros and continued operations in northeastern Mexico, establishing American military dominance in the region and demonstrating the effectiveness of Taylor's army in the opening phase of the Mexican–American War.
The Mexican-American War (1846–1848) grew from the annexation of Texas (1845) and a disputed border between Texas and Mexico at the Rio Grande. President James K. Polk ordered US troops under General Zachary Taylor into the contested zone; after a skirmish that killed American soldiers, Congress declared war in May 1846. US forces won a series of engagements — Palo Alto, Monterrey, Buena Vista — before General Winfield Scott led an amphibious landing at Veracruz and an overland campaign to Mexico City, which fell in September 1847. The Treaty of Guadalupe Hidalgo (February 1848) transferred California, Nevada, Utah, most of Arizona and New Mexico, and parts of Colorado and Wyoming to the United States in exchange for $15 million and assumption of $3.25 million in claims — roughly 525,000 square miles, a 67 percent expansion of US territory. The war's outcome immediately reopened the slavery question: the Wilmot Proviso, debated throughout the war, proposed banning slavery from any territory acquired from Mexico, foreshadowing the sectional crisis of the 1850s.
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