When was the Mexican-American War?

When was the Mexican-American War? Video

The Mexican-American War, lasting from 1846 to 1848, saw the United States transform from a defensive power to an offensive power in the international arena. The war accelerated the country’s westward expansion – yet also had profound consequences for the national controversy of slavery. Today, we’ll take a closer look at its causes, results, and implications going forward.

Causes of the Mexican-American War

The U.S. had been on a frenetic westward land grab in the first few decades of the 19th century, opening up new opportunities for American settlers – but also displacing large numbers of Native Americans and Mexicans in the process. Much of this divisory process of expansion was the work of Andrew Jackson. He had invaded Florida and helped bring that territory into the U.S. in 1818. As president throughout much of the 1830s, Jackson spurred the westward emigration of American settlers supporting the expansion of slavery into U.S. western territories.

By the time of the election of 1844, Democratic politician James K. Polk was itching to continue the Jacksonian legacy. Shortly after being elected, he connived to get Texas – founded on territory seized from Mexico by rebellious American settlers – annexed as part of the U.S. in 1845, as a slave state. This was all enormously controversial, since Mexico deemed the U.S. annexation of Texas illegal and broke off formal diplomatic relations over the annexation. But Polk was determined to gobble up more Mexican territory. He knew the Mexican government, plagued by domestic seizures of power, might be compliant to offers to buy some of their northern lands.

Polk not only wanted to absorb more Mexican holdings into Texas – he also wanted to buy the rights to New Mexico and California. Presidential envoy John Slidell got approval from his Mexican counterparts to begin negotiations in September 1845. His offer? The U.S. would assume the burden of compensating all American businessmen with claims against Mexico, in return for receiving from Mexico the territory between the Nueces River (the official Texas-Mexico border) and the Rio Grande, as well as New Mexico and California, for $25 million.

But political storms were raging in Mexico, set off by Slidell’s arrival in the country. Word began spreading that then-President José Joaquin Herrera was about to sell his country down the river at the negotiating table. To forestall an appearance of weakness – and protect himself against a coup – Herrera canceled his meeting with Slidell. But the negative publicity against Herrera was already out of the bag. Major General Mariano Paredes marshaled enough military and Catholic Church support to pull off a coup that ousted Herrera and made himself president. Looking to shore up a wall of support that Herrera had been too lax in recognizing, Paredes promised a much tougher stance against the Americans.

With the Mexican government emphatically refusing to negotiate with the Slidell mission, Polk threw subtle tactics to the side and orchestrated diplomatic and military incitements to war. In March of 1846, he sent General Zachary Taylor with a detachment of troops to the Rio Grande, where they occupied Corpus Christi and built a fort. They did this so they would be fired on by Mexicans and give Polk a reason to invade Mexico. And that’s exactly what happened.

With Taylor dispatched south, Polk also provoked the Mexicans on the California coast by conducting naval exercises, designing a double threat before war was even declared. The chain of events followed a path that played right into Polk’s hands. President Paredes sent out a decree on April 23rd pointing out that Mexico was being invaded, thereby legitimizing defensive actions to repel the invaders. Two days later in enactment of this order, Mexican troops ambushed an American division under the command of Captain Seth B. Thornton, serving under Taylor. On May 3rd, the Mexicans advanced with 5,000 men and opened fire on the American fort near Corpus Christi, sparking the battles of Palo Alto and Resaca de la Palma, in which Taylor’s army defeated his Mexican opponents. Ten days later, President Polk, with the approval of Congress, issued a declaration of war against Mexico.

Amid popular excitement for the war – it was, after all, the first time the United States was invading another sovereign country – divisions over the war were crystallizing across the board. Former president John Quincy Adams, then serving in Congress, was among the 14 congressmen who voted against Polk’s war declaration, which carried with 174 votes in support. Soon many other congressional Whigs in addition to Quincy Adams were questioning the constitutionality of the war. Junior Illinois Representative Abraham Lincoln was among them.

The war and its proper limitations also divided Polk’s Democratic Party, often along Northern-Southern lines. Late in the summer of 1846, just a few months after war had been declared, Pennsylvania Democratic congressman David Wilmot snuck in an addendum – or “proviso” – into one of the president’s appropriations bills. That proviso mandated the general banning of slavery in any lands gained through war with Mexico. The addendum passed the House but met with stiff opposition in the Senate. Senator John C. Calhoun of South Carolina argued that it infringed on Southern slaveholders’ right to property in settling new lands – claiming that slaves were property. The Wilmot Proviso was defeated in the Senate, but on the other side of the controversy, Whigs in the North smelled blood and would eventually take it up as a weapon against entrenched Democratic power.

Though the U.S. had actually begun hostilities by sending troops into Mexican territory, Polk and Taylor were able to spin the narrative as being Mexican attacks on American Texas, thereby requiring a defensive war against a Mexican aggressor. It was a shrewd political calculation, because it infused the American military effort with a strong national spirit. In addition to the regular army, 30 American volunteer regiments were called up and eventually deployed in the hostilities, many of them fighting with distinction.

Military Campaigns

For the Mexicans to have a chance of winning, they needed to turn the tables by striking north, invading U.S. territory, and falling on the Americans from the rear. However, that was not the course of action they took. Instead, the Americans descended on Mexico in a dual land-and-sea attack from the west and the east – one campaign focusing on the Gulf of Mexico, the other on Mexico’s Pacific coast. The Mexican government brought General Antonio López de Santa Anna, himself a former Mexican president, out of exile to lead all Mexican military forces. Santa Anna kept his end of the bargain – and then, realizing that no one could stop him, decided to become president again.

But even with a seasoned general leading the Mexican war effort as a virtual dictator, the Americans never really lost the initiative and went from victory to victory. After checking a Mexican advance at the Rio Grande, General Taylor marched south, taking Monterrey by storm and later decisively defeating Santa Anna at Buena Vista. General Winfield Scott pressed even deeper into Mexico – seizing the port of Vera Cruz and then marching all the way to Mexico City, taking it by assault in a series of battles. By October 1847, he was military governor of the city, marking the first time U.S. troops had captured a foreign capital. Meanwhile, U.S. forces were delivering a series of knockout blows in the Pacific theater, including the capture of Mazatlán in November 1847, thereby cutting off a major source of supplies for the Mexican troops and forcing peace negotiations.

Nicholas P. Trist, representing the American side in those negotiations, would eventually reach a settlement with the Mexicans for the U.S. to pay the sum of $15 million for an extension of the Texan border to the Rio Grande, and an additional land cession that eventually comprised the American southwest. The deal was finalized in the Treaty of Guadalupe Hidalgo, signed in February 1848 in the vicinity of Mexico City.

Though on the surface it was widely popular in the U.S., the Mexican-American War was nonetheless controversial. President Polk provoked it under a fraudulent pretext and pursued it as a war of conquest. The entry of so much new territory into the country in 1848, meanwhile, begged the question of whether these lands would be slave or free. Texas, enlarged from the war, had become a powerful slave state. But pro-slavery and anti-slavery activists hotly contested the labor rules in the other western territories. David Wilmot’s addendum brought this dispute into the open, fueling the growing animosity over the slavery issue that spawned temporary compromises and, eventually, civil war.

Now, before we go, let’s look at a review question to see what you can remember:

Review Questions

President Polk’s prosecution of war against Mexico resulted in

  1. A two-pronged assault by Generals Zachary Taylor and Winfield Scott on Mexican territory from the east and west.
  2. General Taylor’s eventual garnering of support from the Democratic Party to become nominated as its candidate for president.
  3. A congressional proviso authored by David Wilmot in 1846 which called for the banning of slavery in any territory annexed as a result of the Mexican-American War.
  4. All of the above.
  5. A. and B.
  6. B. and C.
The correct answer is F.

An assault on Mexico from the east and west as well as the Wilmot Proviso.


Thanks for watching, and happy studying!


Return to History Videos



by Mometrix Test Preparation | Last Updated: February 2, 2024