In a blatantly provocative act, American forces under General Zachary Taylor were ordered to move southward from their position on the Nueces River to the Rio Grande. On April 26, 1846 a band of Mexican soldiers crossed the river and attacked a smaller detachment of American soldiers, killing 16.
Two other Mexican attacks occurred north of the Rio Grande, one on May 8 at Palo Alto, and the other on May 9 at Resaca de la Palma, both in Texas.
Mexican spirits had been very high. It must be remembered that the United States was not a major world power in 1846. The Mexicans were confident that they would be able to retake Texas and were hopeful of conquering such port cities as Mobile and New Orleans. Despite high expectations, the Mexican forces were poorly led and inadequately supplied, probably dooming their chances from the start. The defeats at Palo Alto and Resaca de la Palma deeply shook public confidence in the Mexican leadership.
War fever gripped parts of the United States, particularly in the South and Mississippi Valley. Volunteers hoped unrealistically for vast wealth from Mexican gold (which had long since been removed by the conquistadores). Other sections of the country were not enthusiastic and realized that the addition of new territory would reheat the slavery debate. Young Abraham Lincoln, then serving in the House of Representatives, cast his vote against the war. There also was widespread opposition in New England, where such individuals as Ralph Waldo Emerson and Henry David Thoreau expressed their opposition.
Polk claimed that Mexico had invaded the United States and that a state of war existed. Congress passed a formal declaration on May 13, 1846.
Early military action followed in northern Mexico and in the West.