After crushing Texan resistance at the Alamo and at Goliad, Mexican Gen. Antonio Lopez de Santa Anna was confident that Sam Houston’s ragtag army of 800 volunteers posed no threat to his superior force.
So, as Santa Anna’s army bedded down near San Jacinto on April 20, 1836, it posted few lookouts. Early the next morning, when Houston’s force attacked, it caught Santa Anna’s men sleeping. Within 18 minutes, the Texans had killed or captured Santa Anna’s army, including Santa Anna himself, forcing the general to withdraw all Mexican troops from Texas.
Because of his overconfidence, Santa Anna had, in minutes, ceded some 260,000 square miles of territory, which Mexico never would regain.
— Adapted from Eighteen Minutes: The Battle of San Jacinto and the Texas Independence Campaign, Stephen Moore, Republic of Texas.