Outbreak of the Mexican–American War over the disputed Texas border

Background. Tensions arose over the Texas border, with Mexico claiming the Nueces River as the boundary while the United States asserted the Rio Grande. President James K. Polk ordered General Zachary Taylor's Army of Occupation to the Rio Grande in early 1846. Mexican President Mariano Paredes declared he would uphold Mexican territory to the Sabine River, leading to a standoff. The disputed territory lay between the two rivers, setting the stage for conflict.

Outbreak of Hostilities. On April 25, 1846, a Mexican cavalry detachment attacked a small U.S. patrol commanded by Captain Seth Thornton in the contested territory. This event, known as the Thornton Affair, occurred west of Taylor's camp along the Rio Grande. Polk used this attack to convince Congress to declare war. The siege of Fort Texas began shortly after, on May 3, 1846, further escalating the situation.

First Major Battle. The Battle of Palo Alto was the first major battle of the war, fought on May 8, 1846. It took place on disputed ground five miles from modern-day Brownsville, Texas. General Zachary Taylor defeated Mexican troops commanded by General Mariano Arista. This victory drove Arista's troops out of Texas and established Taylor's reputation as a military leader.