In 1846, graduates of West Point Military Academy were deployed to fight in the Mexican-American War (1846 to 1848). Less than 20 years later, many now wearing different uniforms, they would meet again as adversaries in the American Civil War (1861 to 1865), leading some later historians to refer to the earlier conflict as a 'training ground' for the latter.

To characterize the Mexican-American War as only a training ground for the officers who would later serve on both sides in the Civil War is a simplification and a disservice to all who fought between 1846 and 1848, but, at the same time, there is truth to the label. The Mexican-American War provided the theater in which many of the most famous Civil War generals learned the art of warfare firsthand, and they made use of those lessons later to great effect. American Battlefield Trust sums the situation up concisely:

It was during the Mexican-American War that these men developed the strategy and tactics that would dominate the Civil War. Some of these strategies and tactics include:

Additionally, many of these prominent generals became acquainted with one another through their service in the war. The information learned about one another, including temperament, battle style, and character, played vital roles on future battlefields.

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These young officers also all learned from the same two mentors: General Zachary Taylor (1784 to 1850) and General Winfield Scott (1786 to 1866), the supreme commanders of the US troops in the Mexican-American War. Scott would exert significant influence over the young Robert E. Lee, while Ulysses S. Grant would learn more from Taylor, even emulating his casual style of dress, but many of the later Civil War officers learned valuable lessons from one or both of these men.

A complete list of all the Civil War luminaries who first saw action in the Mexican-American War is beyond the scope of this article, but it includes Jefferson Davis, future President of the Confederacy; Joseph Hooker, the Union general who lost the Battle of Chancellorsville; George Pickett, the Confederate general famous for his doomed charge at the Battle of Gettysburg. There was also Albert S. Johnston, Ambrose Burnside, John Bell Hood, A. P Hill, Gideon J. Pillow, Winfield Scott Hancock, and Lewis Armistead – among many others – fighting on the same side before they came to fight each other. Hancock and Armistead were best friends before they came to face each other as enemies, Hancock fighting for the Union and Armistead for the Confederacy.

The following ten are the best-known or, in the case of George H. Thomas, the most overlooked:

Union:

Confederate:

All would serve in the Civil War with distinction, some more effectively than others, and all would draw, to greater or lesser degrees, on what they had learned early on in the Mexico Campaign.

McClellan arrived at his post in October 1846, months after the war had started, and was soon after struck with dysentery and malaria. Upon recovering, he served under General Winfield Scott, leading reconnaissance missions, but he rose through the ranks through his skill as an engineer. After the Battle of Chapultepec, he was promoted to captain and, as an artillery officer, was frequently cited for valor. He later served as Commanding General of the United States Army from 1861 to 1862. His attention to detail, noted in citations during the Mexican-American War, made him overly cautious as a Civil War general, and he repeatedly delayed actions until he had everything just right; so much so that President Abraham Lincoln finally replaced him with Ambrose Burnside, who was far worse.

Meade served under both Winfield Scott and Zachary Taylor as an engineer, taking part in the Battle of Palo Alto, the Battle of Resaca de la Palma, and the Battle of Monterey (where he fought alongside his later nemesis, Robert E. Lee). He was cited for valor and breveted to first lieutenant before the war's end. Meade later served as Major General of the Army of the Potomac from 1863 to 1865, defeating Lee at the Battle of Gettysburg. Meade replaced Joseph Hooker as commander of the Union forces (Hooker had replaced Burnside) and was the first capable commander in charge of operations before Grant became general-in-chief.

Grant first served under Zachary Taylor as a quartermaster but then saw combat at the Battle of Palo Alto and at the Battle of Resaca de la Palma. At the Battle of Monterey, he impressed his commanding officer by delivering messages past enemy snipers by hanging off the side of his horse. He was then transferred to troops under Winfield Scott and, later, at San Cosme, ordered a howitzer taken apart and reassembled in a church tower, from which he fired on enemy troops. His resourcefulness in battle was noted, and he was breveted to captain and cited for valor. Grant later became general-in-chief of the United States Army from 1864 to 1869 and the 18th president of the United States from 1869 to 1877.

Sherman arrived in the territory of Alta California in January 1847, assigned to an administrative post. Scholar Martin Dugard notes: "Sherman was constantly one remove from the action… never got the chance to prove himself in battle" (147). He would certainly do so later, however, serving under Grant between 1862 and 1864 and then as head of the Military Division of the Mississippi, commanding the Western theater of the war. He is best known for his "March to the Sea", conquering Georgia and the Carolinas through a scorched earth policy. He succeeded Grant as commanding general of the United States Army in 1869 and afterwards enacted harsh policies toward the Plains Indians of the American West, which, like his friend George Armstrong Custer, tarnished his reputation as a brilliant Civil War general.

Thomas was a brilliant artillery officer who saw action at the battles of Resaca de la Palma, Fort Brown, Monterey, and Buena Vista under Zachary Taylor. He was respected by his men and superior officers for his calm demeanor under fire and precision in orchestrating artillery barrages. He served alongside his later adversary, Braxton Bragg, and rose quickly through the ranks by the end of the war. In 1863, his ability to remain calm and decisive in combat earned him the nickname "the Rock of Chickamauga" when he held his ground in defense at the Battle of Chickamauga against Bragg. He would later exhibit the same calm he had developed in Mexico in the Franklin-Nashville Campaign of 1864, decisively defeating General John Bell Hood.

Lee arrived in Veracruz in March 1847 as part of Winfield Scott's forces. He led reconnaissance missions and fought in the battles of Cerro Gordo, Contreras, Churubusco, and Chapultepec. He was regularly promoted throughout the conflict and was breveted a captain of engineers. He learned a great deal from Scott, including the fact that, in facing a numerically superior enemy, one could still win through creative tactics and skillful use of terrain. He met Ulysses S. Grant at least once, but they fought in different engagements. He would be appointed commanding general of the Confederate States Army, leading the Army of Northern Virginia from 1862 to 1865. After the Mexican-American War, the next time he met Grant face-to-face was when he surrendered at Appomattox Court House in April 1865.

Thomas Jonathan "Stonewall" Jackson served as an artillery officer under Zachary Taylor, participating in the Siege of Veracruz and the battles of Contreras, Chapultepec, and Mexico City. At Chapultepec, he refused to obey what he considered a 'bad order' to withdraw and was later proven correct. His refusal to back down or retreat later contributed to his nickname, when, as a Confederate officer, he held his line at the First Battle of Bull Run in 1861, "standing like a stone wall." He was accidentally shot by his own pickets at the Battle of Chancellorsville in 1863, later dying from his wounds. The loss of Jackson, Lee's "right-hand man," is thought to have played a significant part in Lee's defeat at Gettysburg later that year.

Longstreet began service in the Mexican-American War under Zachary Taylor, fighting in the battles of Palo Alto, Resaca de la Palma, and Monterey in 1846, during which he led attacks on the enemy and was promoted to first lieutenant. At Chapultepec, when he was carrying the regimental colors, he was wounded and handed the flag to Lieutenant George Pickett, with whom he would later serve in the Civil War, notably at the Battle of Gettysburg. In 1847, he was transferred to Winfield Scott's command and fought in the Battle of Churubusco, which opened the way to Mexico City and the end of the war. Longstreet left the Mexican-American War as a major and would later become a Lieutenant General of the Confederate Army, leading troops from 1861 to 1865.

Pierre Gustave Toutant-Beauregard served under Winfield Scott as a captain of engineers at the battles of Contreras and Churubusco. At the Battle of Chapultepec, he objected to the initial plan of attacking the Candelaria Gate of the city in a frontal assault, suggesting a feint at the gate to draw attention and an attack on the Chapultepec fortress. Scott agreed with him, and the plan was a success. Beauregard was noted for his intelligence and clearness early in the Mexican conflict, traits he exhibited later as a Confederate general between 1861 and 1865. He is probably best known for his defense at the Siege of Petersburg in 1864.

Bragg served under Zachary Taylor before the Mexican-American War had even officially begun. He was an artillery officer under Taylor in Texas in 1845 and then fought at Fort Brown and Monterey. At the Battle of Buena Vista, he defeated a large enemy force, vastly outnumbering his own, through his arrangement of artillery, repulsing the attack. His unit was fighting in support of Colonel Jefferson Davis that day, and Davis would later remember him when Bragg was a Confederate general and Davis the President of the Confederacy.

Although the Mexican-American War should be understood as its own conflict, the lessons learned between 1846 and 1848 informed the engagements of the American Civil War. To cite only one example, the tactic of "the use of massive artillery bombardments to weaken an enemy's defenses, which was followed by a large-scale infantry assault," mentioned above, perfectly describes the Confederate barrage on Union forces on 3 July 1863 just before the infantry assault known as Pickett's Charge at the Battle of Gettysburg.

Among the many poignant aspects of the American Civil War is how many of these men who fought and tried to kill each other had once been classmates and brothers-in-arms, and, in some cases, good friends. Dugard, on page 36 of The Training Ground, notes:

In the second week of March , the young officers of West Point gathered to lead a march on Mexico. Some had fought in battles against the Seminoles, violent and bloody affairs involving great loss of life on both sides; yet, in their minds, this conflict marked the first time they were actually marching off to wage war on another nation. Never did it enter the officers' minds that the battlefields of Mexico might teach them the tactics and lessons they would later use to wage war on one another.