McClung served as lieutenant colonel of the
1st Mississippi Regiment during the
Mexican–American War. Originally captain of the Tombigbee Volunteers, Company K, which was named for the
Tombigbee River, the company was organized at
Columbus, Mississippi, and included soldiers recruited from
Monroe and Lowdnes Counties. He was elected to be lieutenant colonel, second in command, defeating major general of the 3rd division of the Mississippi militia John M. Duffield, and the captain of Raymond Fencibles (Company G) Reuben E. Downing. William P. Rogers took over as captain of Company K. McClung's soldiers were frustrated by his decision-making on a steamboat trip up the
Rio Grande in August–September 1846. Having one soldier go missing (he was picked up later) during what was supposed to be a brief stop at a woodlot for steamboat fuel, "McClung posted a guard whenever the
Colonel Cross docked and forbade the men to leave the boat. On one occasion McClung even armed himself with a brace of pistols and threatened to shoot several volunteers attempting to leave the vessel to find some food suitable for their sick friends on board. His reputation as a duelist made it unlikely anyone took up his challenge. Officers, on the other hand, could come and go freely. The sight of them and their servants going ashore at stops to buy food from the local inhabitants angered the enlisted men." Enlisted men, even the sick, were kept on the deck while officers and their slaves were permitted to enter the cabins. A soldier "N" from the Carroll County Volunteers (Company D) wrote of McClung, "He is the most heartless man I ever saw." The Mississippi Rifles arrived in
Carmargo and camped there until September 7, leaving behind them McClung's trunk, which could not fit on the progressing baggage train, and a disorderly camp with abandoned "tents, kettles, and even newly issued rifles" as well as Lieutenant Bostick, who "had been left by himself to care for nearly 100 sick men without any arrangements made for their provisions." McClung commanded the right wing of the Mississippi Rifles at the
Battle of Monterey, which included companies C, E, G, and K (Vicksburg Southrons, State Fencibles, Raymond Fencibles, and Tombigbee Volunteers). McClung was said to be the first man over the wall at La Tenería (; also known the Black Fort), and "close behind McClung were Lts. William Purnell Townsend and William H. H. Patterson and Pvt. Edward Gregory, all members of the Tombigbee Volunteers." He and his men were assaulting the second Mexican defensive line when he was struck by an enemy
minié ball that "had entered the left hip, after tearing away a portion of the hand which rested on his scabbard, and passed out near the region of the spine." McClung spent the rest of the day wounded in a ditch; his absence was recognized at dusk and a group of his men found him and carried him to safety under a combined fusillade of bullets and rain. He lost two fingers by surgical amputation, the other two were rendered useless by the wound through the hand, only his thumb remained useful. The battle kicked off a long-running feud between McClung and Mississippi politician
Jefferson Davis. In the retelling of one historian, "In February 1847, Davis became involved in a serious dispute with his second-in-command, Major Alexander McClung, and with Colonel
William Campbell of the First Tennessee, over who really deserved credit for the success of the attack at Buena Vista. Beside the falling out with his second-in-command, Major McClung, several officers complained about Davis' imperious nature. Davis for his part was so concerned about his public image and so determined to promote himself as a
war hero that while home on leave he wrote an angry letter to a Mississippi newspaper that demanded that the editor retract a story that gave credit for the victories to the First Tennessee and its commander, Colonel William Campbell."
Honors The 2nd Mississippi Rifles mustered for duty in 1847 at Camp McClung, named in honor of the wounded McClung, which was set up north of Vicksburg along the river below the
Walnut Hills. In 1847, citizens of Columbus presented McClung with a ceremonial sword inscribed "'Presented to Lieut. Col. Alex'r. McClung, by the citizens of Lowndes County, Miss., for his gallantry at the battle of Monterey.' On the sword are the words uttered by the gallant Colonel, when making his charge on the Mexican defences, 'Tombigbee Volunteers, follow me.'" As of 1904 the sword was in the possession of Mrs. Harrison T. McClung of St. Paul, Minnesota. == ''Chargé d'affaires'' in Bolivia ==