Ownership issues aside, the U.S. had already de facto control over much of Texas by the time they asked the Mexican government permission to buy the lands. Spain allowed American settlers in the region, so by the time Mexico declared its independence and claimed Texas for itself, thousands of American families were already living there. The U.S. dug in, prepared to fight to defend its people, if necessary.

On this day, March 27, in 1846, a group of several thousand Mexican soldiers approached the Nueces river in southern Texas and surround the American garrison in Fort Brown, marking the start of hostilities in the Mexican-American war.

The fort was hastily constructed – most of it was just earthwork walls 10 feet high – but it held well under artillery bombardment. Unable to take the fort by cannon fire and unwilling to risk heavy casualties by a frontal assault, Mexican commander Mariano Arista laid siege, in hopes of being able to prevent American reinforcements from reaching the fort. But a relief force led by General Zachary Taylor routed the Mexican army and relieved the fort.