Mexican American War was the training ground for the Civil War Officers.
https://en.wikipedia.org/wiki/Mexican%E2%80%93American_War
Impacts on the American Civil War[edit]
Engraving of young Grant in uniform
Second lieutenant Ulysses S. Grant, one of the many officers in the U.S. Army in the U.S.-Mexican War to serve in the Civil War
Many of the military leaders on both sides of the American Civil War were trained at the U.S. Military Academy at West Point and had fought as junior officers in Mexico. This list includes military men fighting for the Union: Ulysses S. Grant, George B. McClellan, William T. Sherman, George Meade, William Rosecrans, and Ambrose Burnside. Military men who joined the Southern secessionists of the Confederate States of America were Robert E. Lee, Stonewall Jackson, James Longstreet, Joseph E. Johnston, Braxton Bragg, Sterling Price, and the future Confederate President Jefferson Davis. Both sides had leaders with significant experience in active combat in strategy and tactics, likely shaping ways the Civil War conflict played out.
President Ulysses S. Grant, who as a young army lieutenant had served in Mexico under General Taylor, recalled in his Memoirs, published in 1885, that:
Generally, the officers of the army were indifferent whether the annexation was consummated or not; but not so all of them. For myself, I was bitterly opposed to the measure, and to this day regard the war, which resulted, as one of the most unjust ever waged by a stronger against a weaker nation. It was an instance of a republic following the bad example of European monarchies, in not considering justice in their desire to acquire additional territory.[165]
Grant also expressed the view that the war against Mexico had brought punishment on the United States in the form of the American Civil War:
The Southern rebellion was largely the outgrowth of the Mexican war. Nations, like individuals, are punished for their transgressions. We got our punishment in the most sanguinary and expensive war of modern times.[166]
This view was shared by the philosopher Ralph Waldo Emerson, who towards the end of the war wrote that “The United States will conquer Mexico, but it will be as the man swallows the arsenic, which brings him down in turn. Mexico will poison us.”[167]
“An Available Candidate: The One Qualification for a Whig President.” Political cartoon about the 1848 presidential election which refers to Zachary Taylor or Winfield Scott, the two leading contenders for the Whig Party nomination in the aftermath of the Mexican–American War. Published by Nathaniel Currier in 1848, digitally restored.
Despite initial objections from the Whigs and abolitionists, the war would nevertheless unite the U.S. in a common cause and was fought almost entirely by volunteers. The army swelled from just over 6,000 to more than 115,000. The majority of 12-month volunteers in Scott’s army decided that a year’s fighting was enough and returned to the U.S.[168]
Veterans of the war were often broken men. “As the sick and wounded from Taylor’s and Scott’s campaigns made their way back from Mexico to the United States, their condition shocked the folks at home. Husbands, sons, and brothers returned in broken health, some with missing limbs.”[169]
For years afterward, U.S. veterans continued to suffer from the debilitating diseases contracted during the campaigns. The casualty rate was thus easily over 25% for the 17 months of the war; the total casualties may have reached 35–40% if later injury- and disease-related deaths are added.[citation needed] In this respect, the war was proportionately the most deadly in American military history.[citation needed] Overall, approximately 1.5% of U.S. soldiers were killed in the fighting and nearly 10% died of disease; another 12% were wounded or discharged because of disease, or both.[citation needed]
During the war, political quarrels in the U.S. arose regarding the disposition of conquered Mexico. A brief “All-Mexico” movement urged annexation of the entire territory. Veterans of the war who had seen Mexico at first hand were unenthusiastic.[citation needed] Anti-slavery elements opposed that position and fought for the exclusion of slavery from any territory absorbed by the U.S.[170] In 1847 the House of Representatives passed the Wilmot Proviso, stipulating that none of the territory acquired should be open to slavery. The Senate avoided the issue, and a late attempt to add it to the Treaty of Guadalupe Hidalgo was defeated.
The Treaty of Guadalupe Hidalgo was the result of Nicholas Trist’s unauthorized negotiations. It was approved by the U.S. Senate on March 10, 1848, and ratified by the Mexican Congress on May 25. Mexico’s cession of Alta California and Nuevo México and its recognition of U.S. sovereignty over all of Texas north of the Rio Grande formalized the addition of 1.2 million square miles (3.1 million km2) of territory to the United States. In return the U.S. agreed to pay $15 million and assumed the claims of its citizens against Mexico. A final territorial adjustment between Mexico and the U.S. was made by the Gadsden Purchase in 1853. The sale of this territory was a contributing factor in the final fall of Santa Anna in Mexico for having sold Mexican patrimony.
As late as 1880, the “Republican Campaign Textbook” by the Republican Congressional Committee[171] described the war as “Feculent, reeking Corruption” and “one of the darkest scenes in our history—a war forced upon our and the Mexican people by the high-handed usurpations of Pres’t Polk in pursuit of territorial aggrandizement of the slave oligarchy.”
The war was one of the most decisive events for the U.S. in the first half of the 19th century. While it marked a significant waypoint for the nation as a growing military power, it also served as a milestone especially within the U.S. narrative of Manifest Destiny. The resultant territorial gains set in motion many of the defining trends in American 19th-century history, particularly for the American West. The war did not resolve the issue of slavery in the U.S. but rather in many ways inflamed it, as potential westward expansion of the institution took an increasingly central and heated theme in national debates preceding the American Civil War. Furthermore, in doing much to extend the nation from coast to coast, the Mexican–American War was one step in the massive migrations to the West of Anglo Americans, which culminated in transcontinental railroads and the Indian wars later in the same century.
Without necessarily calling into question the facts presented in that Wikipedia entry, I do get the sense that it was in some degree edited by folks who don’t exactly love America, and might prefer that we give certain formerly Mexican territories back.