Coming off two quick victories at Palo Alto and Resaca de la Palma in May, General Zachary Taylor prepared to launch an invasion of Mexican states below the Rio Grande in the summer of 1846. Establishing a base of operations at Camargo, Taylor spent most of the summer waiting for reinforcements. In July, militia regiments which had been hastily organized following the War Department's call for volunteers began to arrive. The troops were generally ill-supplied, ill-prepared, and lacking in discipline. Failing to take adequate sanitation precautions, they soon began to suffer from dysentery and other diseases. By the time Taylor began his march south in September, 1,500 had died.
On the Mexican side, Antonio López de Santa Anna, having recently returned from exile and regained the presidency, ordered Mexican general Pedro de Ampudia, who had assumed command of the Army of the North following the court-martial of General Mariano Arista, to withdraw to Saltillo in order to establish a defensive position. Ampudia decided, however, to make a stand against the U.S. forces at Monterrey, Mexico's largest northernmost city. Surrounded by low foothills in a valley of the Sierra Madres, Ampudia believed the city of 20,000 inhabitants to be a more strategically important and more readily defensible location.
Numbering 6,500 regulars and volunteers, Taylor's army reached the outskirts of the city on September 19. On September 21, Taylor probed an approach of the city from the east, while sending William Worth, a former commandant of cadets at West Point, on a flanking movement to the southwest to cut off the Mexican army's escape route. Worth succeeded in doing so, and gained steady ground against Mexican hilltop fortifications, seizing Federation Hill on the 21st and the Bishop's Palace the following day. On the evening of the 22nd Ampudia ordered his troops to take up defensive positions within the city, setting the stage for bitter house-to-house fighting on the 23rd. On the morning of September 24, U.S. troops were preparing to renew their assault when Ampudia asked for a parley. With his army running low on ammunition after three days of fighting and discipline beginning to break down, Taylor allowed Ampudia's troops to evacuate the city, taking with them their weapons and one six-gun battery. Since poor communication made it impossible for either general to obtain instructions from his government at short notice, Taylor and Ampudia signed an eight-week armistice, during which Taylor's forces would not advance farther than fifty miles to the south. The agreement further stipulated that it could be rescinded if either government found its terms unsatisfactory.
The reputation of both generals suffered as a result of the battle, at least as far as their respective governments were concerned. Although Ampudia had been allowed to withdraw with his army intact, the need for a resounding victory prompted Santa Anna to take the lead as commander-in-chief of Mexican forces. He would personally take charge of Mexican troops in the field for the remainder of the war. Taylor, meanwhile, though widely hailed as a war hero by the American press, was reprimanded by the Polk administration for failing to demand Ampudia's unconditional surrender. Taylor's new orders from Washington pointedly refrained from congratulating the general, and ordered him to terminate the armistice immediately.
Carney, Stephen A., and Center of Military History. Gateway South: The Campaign for Monterrey. Government Printing Office, 2005.
Dishman, Chris D. A Perfect Gibraltar: The Battle for Monterrey, Mexico, 1846. Norman: University of Oklahoma Press, 2010.
Eisenhower, John S. D. Zachary Taylor. New York: Times Books, 2008.
Johannsen, Robert. To the Halls of the Montezumas: The Mexican War in the American Imagination. New York: Oxford University Press, 1985.