1. Before invading Mexico, the U.S. tried to buy some of its territory.

In late-1845, President James K. Polk sent diplomat John Slidell on a secret mission to Mexico. Slidell was tasked with settling a longstanding disagreement about the border between the two countries, but he was also authorized to offer the Mexicans up to $25 million for their territories in New Mexico and California.

When the Mexicans refused to consider the offer, Polk upped the ante by ordering 4,000 troops under Zachary Taylor to occupy the land between the Nueces River and the Rio Grande—a region Mexico claimed as its own territory. Mexico replied by sending troops to the disputed zone, and on April 25, 1846, their cavalry attacked a patrol of American dragoons. Polk’s opponents would later argue the president had goaded the Mexicans into the fight. 

Nevertheless, on May 13, 1846, Congress voted to declare war on Mexico by an overwhelming margin.

2. The war marked the combat debut of several future Civil War generals.

Along with future presidents Zachary Taylor and Franklin Pierce, the U.S. force in Mexico included many officers who later made their name on the battlefields of the Civil War.

Union Generals Ulysses S. Grant, George Meade and George McClellan all served, as did many of their Confederate adversaries such as Robert E. Lee, Stonewall Jackson and George Pickett. Lee, then a captain in the Army Corps of Engineers, emerged from the war a hero after he scouted passes that allowed the Americans to outmaneuver the Mexicans at the Battles of Cerro Gordo and Contreras.

3. Santa Anna used the war to reclaim power in Mexico.

Most Americans considered Antonio Lopez de Santa Anna a mortal enemy for his actions at 1836’s Battle of the Alamo, but the charismatic general returned to power during the Mexican-American War thanks to a surprising ally: James K. Polk.

Santa Anna was languishing in Cuba when the war began, having been driven into exile after a stint as Mexico’s dictator. In August 1846, he convinced the Polk administration that he would negotiate a favorable peace if he were allowed to return home through an American naval blockade. Polk took the general at his word, but shortly after setting foot on Mexican soil, Santa Anna double-crossed the Americans and organized troops to fight off the invasion. Along with reclaiming the presidency, he went on to lead the Mexicans during nearly all the war’s major battles.

4. Abraham Lincoln was one of the war’s harshest critics.

The invasion of Mexico was one of the first U.S. conflicts to spawn a widespread anti-war movement. Political opponents labeled “Mr. Polk’s War” a shameless land grab, while abolitionists viewed it was a scheme to add more slave states to the Union. Among the more notable critics was freshman Illinois congressman Abraham Lincoln, who took to the House floor in 1847 and introduced a series of resolutions demanding to know the location of the “spot of soil” where the war’s first skirmish took place.

Lincoln maintained that the battle had been provoked on Mexican land, and he branded Polk a cowardly seeker of “military glory.” The so-called “Spot Resolutions” helped put Lincoln on the map as a politician, but they also damaged his reputation with his pro-war constituents. One Illinois newspaper even branded him “the Benedict Arnold of our district.”

5. It included the U.S. military’s first major amphibious attack.

The most significant phase of the Mexican-American War began in March 1847, when General Winfield Scott invaded the Mexican city of Veracruz from the sea. In what amounted to America’s largest amphibious operation until World War II, the Navy used purpose-built surfboats to ferry more than 10,000 U.S. troops to the beach in just five hours. The landings were mostly unopposed by the town’s outnumbered garrison, which later surrendered after an artillery bombardment and a 20-day siege. Having secured Veracruz, Scott’s army launched the war’s final thrust: a six-month, 265-mile fighting march to the “Halls of Montezuma” at Mexico City.

6. A band of Irish Catholics deserted the U.S. and fought for Mexico.

One of the war’s most storied units was St. Patrick’s Battalion, a group of U.S. soldiers who deserted the army and cast their lot with Mexico. The 200-man outfit was mostly made up of Irish Catholics and other immigrants who resented the prejudice they faced from Protestants in the United States.

Under the leadership of an Irishman named John Riley, the “San Patricios” defected and became Santa Anna’s elite artillery force. They served with distinction at the Battles of Buena Vista and Cerro Gordo, but most of their unit was later killed or captured during an August 1847 clash at Churubusco. Following a court martial, the U.S. Army executed around 50 of the soldiers by hanging. Several others were whipped and branded with a “D” for “deserter.” Though scorned in the United States, the San Patricios became national heroes in Mexico, where they are still honored every St. Patrick’s Day.

7. The Battle of Chapultepec gave rise to a famous legend in Mexico.

When they arrived in Mexico City in September 1847, U.S. forces found the western route into the capital blocked by Chapultepec Castle, an imposing fortress that was home to Mexico’s military academy. General Scott ordered an artillery bombardment, and on September 13 his troops stormed the citadel and used ladders to scale its stone façade. Most of the Mexican defenders soon withdrew, but a group of six teenaged military cadets remained at their posts and fought to the last.

According to battlefield lore, one cadet prevented the capture of the Mexican flag by wrapping it around his body and leaping to his death off the castle walls. While Chapultepec was lost, Mexicans hailed the six young students as the “Niños Heroes,” or “Hero children.” They were later honored with a large monument in Mexico City.

8. An American diplomat disobeyed orders to end the war.

As the war inched toward its conclusion in 1847, President Polk sent State Department clerk Nicholas P. Trist south of the border to seal a peace treaty with the Mexicans. Negotiations proceeded slowly at first, and in November 1847 Polk grew frustrated and ordered Trist to end the talks and return home. Trist, however, would do no such thing. Believing he was on the verge of a breakthrough with Mexicans, he disobeyed the President’s order and instead wrote a 65-page letter defending his decision to continue his peace efforts. Polk was left seething. He called Trist “destitute of honor or principle” and tried to have him removed from the U.S. Army headquarters, but he was unable to stop the negotiations.

On February 2, 1848, Trist struck the Treaty of Guadelupe Hidalgo, an agreement in principle to end the war. While Polk reluctantly accepted the deal, he fired Trist as soon as the rogue diplomat returned to the United States.

9. The war reduced the size of Mexico by more than half.

Along with relinquishing all claims to Texas, the Treaty of Guadalupe Hidalgo also forced Mexico to accept an American payment of $15 million for 525,000 square miles of its territory—a plot larger than the size of Peru. The lands ceded by Mexico would later encompass all or part of the future states of California, New Mexico, Nevada, Utah, Arizona, Colorado, Wyoming, Oklahoma and Kansas.

10. It had one of the highest casualty rates of any American war.

The U.S. never a lost a major battle during the Mexican-American War, but the victory still proved costly. Of the 79,000 American troops who took part, 13,200 died for a mortality rate of nearly 17 percent—higher than World War I and World War II.

The vast majority were victims of diseases such as dysentery, yellow fever, malaria and smallpox. According to scholar V.J. Cirillo, a higher percentage of U.S. troops died from sickness during the Mexican invasion than any war in American history. Mexican casualties were also high, with most historians estimating as many as 25,000 dead troops and civilians.

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