“None but an army of heroes could have accomplished the conquest of Mexico.”
—William Loring, 1880

In May of 1846, Congress authorized the formation of a Regiment of Mounted Riflemen, and Loring abandoned law and politics to join the United States Army. Originally created to protect the Oregon Territory, the regiment was soon sent to fight in the war against Mexico. Captain Loring was quickly promoted to regimental major, even before the unit saw action. On March 9, 1847, the Rifles waded ashore near Vera Cruz, part of the first major amphibious assault by the U.S. Army. The regiment had lost its horses at sea, and was forced to fight as infantry (a situation faced by the Rough Riders in nearby Cuba fifty years later). During the battle of Cerro Gordo, Loring and the Rifles stormed the heights, helping decisively defeat Santa Anna’s forces. In his report of the battle, General Winfield Scott wrote:

A portion of the First Artillery...the Third Infantry...and the Mounted Riflemen under Major Loring, all under the temporary command of Colonel Harvey of the Second Dragoons. The styled execution was brilliant and decisive. The Brigade ascended the long and difficult slope of Cerro Gordo, without shelter and under the tremendous fire of artillery and musketry, with the utmost steadiness, reached the breastworks, drove the enemy from them, and planted the colors of all units with the enemy flag still flying. After some minutes of sharp fighting, they finished the conquest with their bayonets. It is a most pleasing duty to say that the highest praise is due Harvey, Plympton, Loring, and Alexander, their gallant officers and men for their brilliant service.

On September 13, the Rifles took part in the storming of the Castle of Chapultepec. With the fall of Mexico’s military academy, one last obstacle to the halls of Montezuma remained: the city’s gates. Rushing forward, William Loring in the lead, the Rifles commenced the assault on Belan Gate. But even as the Americans breached the final barrier, a Mexican bullet struck Loring, shattering his arm. Dr. H.H. Steiner described what followed:

Loring laid aside a cigar, sat quietly in a chair without opiates to relieve the pain, and allowed the arm to be cut off without a murmur or a groan. The arm was buried on the heights by his men, with the hand pointing towards the City of Mexico.

With U.S. soldiers inside the city’s gates, Santa Anna abandoned Mexico’s capital, effectively ending the war. Loring was brevetted colonel for gallant and meritorious conduct in the battle. During the campaign for Mexico City, Loring became friends with Lieutenant Ulysses S. Grant. The two would meet again on the field of battle outside Vicksburg, and one final time in a place far from North America. In fact, Loring’s life was ever after tied to other veterans of the Mexican War, including Jefferson Davis, Robert E. Lee, Thomas “Stonewall” Jackson, John Pemberton, Joseph Johnston, Henry Hopkins Sibley, and Charles Pomeroy Stone. But for now, Oregon awaited. Despite his wound, Loring decided to remain in the army.