Polk, eager to acquire California and the Southwest, had sent General Zachary Taylor's army to the Rio Grande, into land Mexico considered its own after the annexation of Texas. When Mexican cavalry attacked a patrol, Polk told Congress that Mexico had "shed American blood upon the American soil". The House approved the declaration by 174 to 14 after brief debate. The war ended in 1848 with Mexico ceding about half its territory, including California, for $15 million.
The United States had annexed Texas in 1845, and the two countries disagreed about its border: Texas claimed the Rio Grande, Mexico the Nueces River farther north. Polk ordered Taylor into the strip between them. On April 25, 1846, Mexican cavalry ambushed a party of American dragoons, killing 11. The news took about two weeks to reach Washington.
On May 11 Polk sent his war message. The House of Representatives took about two hours to approve a bill funding the war, with a preamble declaring that war existed "by the act of the Republic of Mexico", and the Senate followed. Polk signed it on May 13. Among the few dissenters was former president John Quincy Adams, who called the war unjust.
Many Americans agreed with him. The young Whig congressman Abraham Lincoln, elected that year, later challenged Polk to show the exact "spot" where American blood had been shed. The Treaty of Guadalupe Hidalgo in 1848 transferred California, Nevada, Utah and most of Arizona and New Mexico, and the question of slavery in the new territory helped lead to civil war.
Key people: James K. Polk, Zachary Taylor, John Quincy Adams, Abraham Lincoln