The Early Battles in Texas.
When Fort Texas was besieged, Zachary Taylor led a force to relieve the garrison. This force was engaged by a larger Mexican force at the Battle of Palo Alto. The superiority of the American artillery drove the Mexicans back, and they established fortified defenses at Resaca de la Palma. During the battle there the following day the Mexican army was again forced to retreat, after fierce combat, and the retreat became a rout, with the fleeing Mexicans struck repeatedly by pursuing American cavalry. The Mexicans were driven across the Rio Grande by American troops, which paused once they reached the river.
On May 11, 1846, President Polk sent a war message to Congress, citing the Mexican actions in the disputed region known as the Nueces strip as acts of war. Congress approved the declaration of war on May 13. One congressman who disputed the President’s statement, which accused Mexico of attacking American troops on American soil, was Abraham Lincoln. Lincoln argued that the attacks occurred against American troops which were on Mexican soil, or at any rate on disputed territory which had formerly been claimed by Spain and later by Mexico. Most American abolitionists opposed the war, believing it was an attempt to increase territories which would allow slavery.
Henry David Thoreau vocally opposed the war and refused to pay taxes which would be used to support the American war effort, an act for which he was jailed. In response Thoreau wrote the essay Civil Disobedience, which was published in 1849, after the war was over. In the essay Thoreau argued against the actions of the government and posited that it was the duty of citizens to rise in opposition to governments which acted without conscience. Throughout Thoreau’s New England, the vocal majority opposed the war and the actions of the government under Polk, spurred by abolitionists who believed the war was about spreading slavery.
Their arguments were soon drowned out by the patriotic celebrations which erupted with the news of Zachary Taylor’s military victories over the Mexicans at Palo Alto and the retreat of the Mexicans across the Rio Grande. Newspapers dispatched journalists to the Mexican border, where they joined the American troops and sent back jingoistic stories of American morale and military superiority. The War Department developed a plan to invade Mexico in the west using troops from Fort Leavenworth and the Jefferson Barracks in Missouri, as well as a simultaneous invasion by the troops under Zachary Taylor. A US Naval squadron was sent to the Pacific to support the western invasion.
In Mexico, the defeat of the Mexican troops at Palo Alto and Resaca de la Palma led to the resurgence in power of military hero Antonio Lopez de Santa Anna, who returned from Cuban exile, renounced any political ambitions, and offered his services as a military commander. Santa Anna was encouraged to return to Mexico by US agents sent by President Polk, who offered the former dictator a bribe of $2 million to be paid upon Mexican cession of California. Once Santa Anna reached Mexico City, after being allowed to pass through the American blockade, he reneged on all of his promises and took command of the Mexican Army, declared himself president, and began operations against the American invasion.