The Mexican-American War and the Completion of the Continent

The disputed border between the Nueces and Rio Grande rivers, left unresolved by the annexation of Texas, became the immediate trigger of war: President Polk sent troops under General Zachary Taylor into the contested zone, and after an armed clash on April 25, 1846, presented the incident to Congress as a Mexican invasion of American soil. Congress declared war on May 13, 1846, over the notable opposition of a young Whig congressman, Abraham Lincoln, who publicly challenged the administration's account of where the first clash had actually occurred through his "Spot Resolutions."