In the Battle of Monterrey (September 21–24, 1846) during the Mexican–American War, General Pedro de Ampudia and the Mexican Army of the North was defeated by the Army of Occupation, a force of United States Regulars, Volunteers and Texas Rangers under the command of General Zachary Taylor. The hard-fought urban combat led to heavy casualties on both sides. The battle ended with both sides negotiating a two-month armistice and the Mexican forces being allowed to make an orderly evacuation in return for the surrender of the city. Background Following the Battle of Resaca de la Palma, Taylor crossed the Rio Grande on 18 May, while in early June, Mariano Arista turned over command of what remained of his army, 2,638 men, to Francisco Mejia, who led them to Monterrey. On 8 June, United States Secretary of War William L. Marcy ordered Taylor to continue command of operations in northern Mexico, suggested taking Monterrey, and defined his objective to "dispose the enemy to desire an end to the war." On 8 August, Taylor established the headquarters for his Army of Occupation in Camargo, Tamaulipas and then in Cerralvo on 9 September with 6,640 men. Taylor resumed the march to Monterrey on 11 September, reaching Marin on 15 September and departing on 18 September In early July, General Tomas Requena garrisoned Monterrey with 1,800 men, with the remnants of Arista's army and additional forces from Mexico City arriving by the end of August such that the Mexican forces totaled 7,303 men. General Pedro de Ampudia received orders from Antonio López de Santa Anna to retreat further to the city of Saltillo, where Ampudia was to establish a defensive line, but Ampudia disagreed, sensing glory if he could stop Taylor's advance. Ampudia's forces included reinforcements from Mexico City totaling 3,140 men: 1,080 men of the Garcia-Conde Brigade (Gen. Jose Garcia Conde) (Aguascalientes and Querétaro Battalions, two squadrons of the 3d Line Cavalry, three guns (3-8 lbs)), a thousand men of the Azpeitia Brigade (Col. Florencio Azpeitia) (3d Line, two squadrons of the Jalisco lancers, two squadrons of the Guanajuato Cavalry Regiment, six guns (8 and 12 lbs.) and an ambulance), 1,060 men of the Simeon Ramirez Brigade (Acting Gen. Ramirez) (3d and 4th Light, three guns (1-8 lbs, 2-12 lbs) and 3 howitzers 7" (Capt. P. Gutierrez and Comdte. A. Nieto)) and an artillery unit, the largely Irish-American volunteers called San Patricios (or