Word reached Washington until May 9, and Polk sent a war message to Congress on the ground that Mexico had, "shed American blood on the American soil".[160][161] The House overwhelmingly approved a resolution declaring war and authorizing the president to accept 50,000 volunteers into the military.[162][163] In the Senate, war opponents led by Calhoun questioned Polk's version of events. Nonetheless, the House resolution passed the Senate in a 40–2 vote, with Calhoun abstaining, marking the beginning of the Mexican–American War.[164]

Course of the war[edit]
Overview map of the war .mw-parser-output .legend{page-break-inside:avoid;break-inside:avoid-column}.mw-parser-output .legend-color{display:inline-block;min-width:1.25em;height:1.25em;line-height:1.25;margin:1px 0;text-align:center;border:1px solid black;background-color:transparent;color:black}.mw-parser-output .legend-text{}  Disputed territory   United States territory, 1848   Mexican territory, 1848  After treaty of Guadalupe Hidalgo
After the initial skirmishes, Taylor and much of his army marched away from the river to secure the supply line, leaving a makeshift base, Fort Texas. On the way back to the Rio Grande, Mexican forces under General Mariano Arista attempted to block Taylor's way as other troops laid siege to Fort Texas, forcing the U.S. Army general to the attack if he hoped to relieve the fort. 