After a number of embarassing defeats and near misses, the Army of the North attempted to retreat south and refit before engaging the seemingly unbeatable American forces under General Zachary Taylor. Near the old fortress town of Monterrey, General Pedro de Ampudia recieved orders from Antonio Lopez de Santa Anna to retreat further to the town of Saltillo,Mexico[?] where Ampudia was to establish a defensive line. But Ampudia, who was hungry for victory and conscious that his men were nearing mutiny through constantly being forced to retreat, refused the order and chose instead to make a stand at Monterrey. Joining Ampudia at this engagement were an elite artillery unit, the largely Irish-American San Patricios (or Saint Patrick's Battalion), in their first major engagement against U.S. forces.
For three days, American forces attempted to take the city without success. Heavy Mexican resistance caused considerable losses in American ranks, and American artillery found itself incapable of penetrating the walls of the numerous fortresses and fortifications in the area. Finally, the Americans began to come close enough to the city to use their only piece of siege artillery, a somewhat antiquated Napoleonic era 32 pound Siege Howitzer, that began to hurl rounds into the central plaza of Monterrey, panicking the local inhabitants.
On the night of the 23rd, a final American push to capture the city walls met with fierce resistance, particularly on the part of the San Patricios. The American line, near to cracking, began a somewhat disorganized retreat. At the same time, Zachary Taylor, determined to win the day, ordered his mortar to begin shelling indiscriminantly. This act finally broke the back of Mexican reisistance, and with American forces in full retreat, Ampudia ordered the white flag of surrender to be flown.
The resulting armistice signed between Taylor and Ampudia had major effects upon the outcome of the war. Taylor was lambasted by Washington, where President James K. Polk insisted that American forces had no authority to negotiate truces, only to "kill the enemy". In addition, his terms of armistice, which allowed the Mexican Army to retreat with battle honors and all of their weapons, were seen as foolish and short-sited by some Americans.
For his part, some have argued that Ampudia had sewn the seeds of defeat for Mexico. Many Mexican soldiers became depressed and disenchanted. In a well fortified, excellently supplied position, an army of twelve thousand Mexican soldiers had nearly defeated the U.S. Army only to be forced to surrender at their moment of triumph. Many felt that their generals simply did not want to win, and desertions and mutiny became widespread problems.
Bauer, K. Jack, "The Mexican War, 1846-1848"