Mexican-American War Mexican-American War. The United States Congress approved the declaration of war on Mexico 165 years ago today. The battle for land ownership caused political controversy and was a precursor to a war that divided America. Mexican-American War origins Texas won independence from Mexico in 1836. Mexico refused to recognize Texas' independence and threatened war if the United States annexed its former territory. Which future American president led Texas' defense? Meanwhile, the United States wanted to expand its landholdings and used the concept of Manifest Destiny to fight for Texas and other land. Mexican-American War declaration May 13 On May 11, 1846, President James Polk announced to Congress that the Mexican army had invaded U.S. soil and had killed 16 American soldiers, in an event known as the Thornton Affair. Congress declared war on May 13 despite allegations that increased territory would be used to increase slaveholding states. Mexican-...