The Natchez Fencibles were not selected to fight in the Mexican War due to a handful of men who failed to muster at
Vicksburg, Mississippi, at the proper time. However on July 1, 1846, during the
Mexican–American War, Quitman was made a Brigadier General of Volunteers. He commanded a brigade under
Zachary Taylor in northern Mexico. After the
Battle of Monterrey, he was sent to join
Winfield Scott's expedition. He led the 2nd Brigade in the Volunteer Division during the
Siege of Veracruz, and on April 14, 1847, he was promoted to major general in the
Regular Army. Following the
Battle of Cerro Gordo, General
Robert Patterson returned to the United States with other Volunteer soldiers whose enlistments had expired. Reinforcements from Veracruz, including about 300 U. S. Marines, were organized into a new brigade under
Samuel E. Watson. Watson's and
James Shields' brigades were designated the 4th Division, with Quitman in overall command. By this point, Quitman had gained a reputation as a competent military commander and enjoyed affectionate respect from volunteer and professional soldiers alike.{{cite book Quitman led his division to the
Valley of Mexico, where he was posted to guard the supply depot, hospital, and horse teams. Frustrated at his supporting role, Quitman was nevertheless ordered to send reinforcements to the front. At the battles of
Contreras and
Churubusco, Shields' brigade was actively engaged, though Quitman was not personally involved. He commanded the southern assault during the
battle of Chapultepec. U.S. Marines of Quitman's division spearheaded the attack, and their involvement in this battle is remembered in the opening line of the
Marines' Hymn. Quitman received the surrender of the citadel in Mexico City. After the
fall of Mexico City, General Scott named Quitman as the military governor of
Mexico City for the remainder of the occupation. He was the only American to rule from within the
National Palace of Mexico. Quitman was a founding member of the
Aztec Club of 1847. He was discharged on July 20, 1848, and served as
governor of Mississippi in 1850 and 1851. ==Filibustering==