After a border skirmish at the Rio Grande on April 25, 1846, A Mexico and the United States were at war, and within a few months Zachary Taylor’s troops had overrun the north, reaching Monterrey and Saltillo. At the same time an expedition under Winfield Scott landed at Vera Cruz and pushed inward along the ancient road of conquest, reaching the gates of Mexico City by August, 1847. Mexican resistance was heroic and determined in spots, but disorganization and poor leadership played havoc with any attempt at national defense. The capital fell and was occupied by the invaders, the Mexican government fled to Querétaro, and an ignominious peace treaty was negotiated and accepted by the helpless Mexicans, though not without serious opposition from the radical (puro) element which favored a last-ditch resistance. With the rapid subjection of the country and the loss of more than half its territory, the once proud and optimistic nation of Iturbide was left stunned; and it turned to bitter reflection upon its paralyzed condition and its flagrant display of weakness when faced by a small and not too efficient force of invaders.
The very independence of Mexico was now threatened. Such an easy victory by a powerful neighbor would mean that Mexico might at any time be absorbed by the United States, especially when there was a movement for that purpose already afoot north of the Rio Grande. The easy optimism of the early days of the republic had now vanished. The shock of military disaster, after the dismal decade of mediocrity and humdrum military revolutions, accentuated a crisis in Mexican thought. Both liberals and conservatives now saw the necessity of imposing radical changes upon the course of independent Mexico. Since the overthrow of the radical Gómez Farias government in 1834, the country had been allowed to drift, and when the Americans invaded, its vigor appeared to be gone. In spite of the presence of a sizable moderate party, the factions became sharply differentiated as they had never been before in the history of the republic, except perhaps for the year 1832. Liberals and conservatives appealed to their traditional programs for solutions to Mexico’s crisis of 1847, and the seeds were sown for a great conflict.