History 103: US History I

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The controversial Mexican-American War lasted from 1846-1848. In this lesson, discover how the Treaty of Guadalupe Hidalgo expanded the southern part of the United States all the way to the Pacific Ocean.

The Roots of the Mexican-American War

The Mexican-American War started in 1846, but its story really begins a quarter of a century earlier. In 1821, Mexico won its independence from Spain. The country was in debt to both its own citizens and Britain. Mexico enticed American citizens into immigrating to the northeastern part of the country, known today as Texas. Mexico had a series of internal battles, and the residents of Texas, most of whom were English-speaking, seceded. In 1836, Texas defeated Mexican General Santa Anna and declared its independence. Mexico never recognized Texas' independence and disputed its borders even after its annexation into the U.S. Mexico insisted the border was at the Nueces River, while Texas insisted it was the Rio Grande.

Polk's War

As soon as the United States annexed Texas, President Polk sent a diplomat, John Slidell, to Mexico to negotiate three things:

1. The border.
2. Debts owed by the Mexican government to U.S. citizens.
3. The purchase of California and New Mexico for as much as $50 million before they sold it to Britain.

Of course, he hoped to weave some of these issues together (such as forgiving the debts if the border were settled at the Rio Grande). But Mexico's own government was in turmoil, and unless Polk had sent someone to undo what Tyler had done, there was probably no way any negotiations would have even occurred. Slidell was sent back to Washington.

But Polk couldn't live with 'no' for an answer. This time, he sent a different kind of negotiator: General Zachary Taylor. The army occupied the disputed land southwest of the Nueces, angering not only the Mexican government but many American citizens as well. When a few soldiers were killed in a skirmish, Polk told Congress that Mexico had shed American blood on American soil. On May 13, 1846, Congress approved the Mexican-American War, but America was deeply divided over the issue. Opponents called it Polk's War.

The Political War

Whigs, like then-Captain Ulysses S. Grant, accused the President of provoking an unjust war against a weaker neighbor - a concept that contradicted the romantic ideals of Manifest Destiny in which democracy would spread because of its own virtue. Abraham Lincoln doubted the provocation and demanded to know the exact spot where American soldiers had been killed. Abolitionists opposed going to war with Mexico because they feared the expansion of slavery. But even some Southern slave owners opposed the war because they didn't want non-whites admitted to the Union. Senator John C. Calhoun of South Carolina said 'We have never dreamt of incorporating into our Union any but the Caucasian race - the free white race…' Then, there was Congressman David Wilmot. He proposed banning all African-Americans - slave or free - from any territory that might be gained at the end of the war.

This so-called Wilmot Proviso was primarily an abolitionist piece of legislation, but Wilmot hoped that it would resonate with the Southern Democrats by reserving all of that land for white yeoman farmers. Calhoun saw through this, of course, and defeated the Wilmot Proviso in the Senate on the grounds that Congress could not legally ban slavery anywhere since the Constitution allowed it. This concept also negated Polk's suggestion of extending the Missouri Compromise line through the Mexican land. Finally, a Michigan senator devised the brilliantly simple solution of popular sovereignty - letting the residents of new territories decide for themselves whether they wanted slavery. Of course, all this negotiation meant nothing at the time, since war had just started. But popular sovereignty persisted as an uneasy solution into the next decade.

The Media War

The court of public opinion was just as divided. An expedited route of information between New York and New Orleans newspapers used the latest technology - the telegraph - to send back glorified details of the heroic actions of General Taylor and his soldiers. The Mexican government had overestimated its ability to fight a war against America, and its forces faced repeated routs and losses of territory.

In reality, it wasn't always good news, but the American media loved to make it seem that way. As the penny press reprinted stories of victory after victory, the public split into two camps: those who saw America as a bully pummeling a weaker neighbor and those who saw an opportunity to take all of Mexico. An essay by the famous transcendentalist Henry David Thoreau persuaded many American citizens to stop paying their taxes in protest. 'Civil Disobedience' landed Thoreau in jail.

The War on the Battlefields

Mexico had overestimated its manpower and weaponry, steadily losing ground to General Taylor and American officers like Ulysses S. Grant, Robert E. Lee, William T. Sherman and others who would later direct the Civil War on both sides. In early 1847, the navy pounded Vera Cruz and marines captured the city. From there, they could move towards Mexico City. The President's political opponents continued to question why America was pushing deeper into Mexico if our goal had been to defend our border. So when marines captured the 'Halls of Montezuma' (Mexico City) in September of 1847, Polk knew it was time to quit.

Meanwhile, back in 1846, Captain John Frémont led American settlers in a rebellion against Mexican authorities in California. They declared the independent 'Bear Flag Republic' and with the navy's help soon asserted that California belonged to the United States. When they met resistance, an army colonel marching through New Mexico came to their rescue and defeated the remaining Mexican forces in California in 1847. Once Mexico City was captured, the peace negotiations included all of California.

The Treaty of Guadalupe Hidalgo

Although President Polk had officially recalled his diplomat from Mexico City, Nicholas Trist proceeded with negotiations. And since the terms were favorable, Polk sent the Treaty of Guadalupe Hidalgo to the Senate for ratification. The main provision of the treaty was America's right to buy about half of Mexico's land (the territories of California and New Mexico) for $15 million. This land has come to be known as the Mexican cession. Additionally, the U.S. would assume $3.25 million in debts owed to U.S. citizens in exchange for settling the Texas border at the Rio Grande.

The Senate approved the Treaty of Guadalupe Hidalgo on March 10, 1848. Though the Wilmot Proviso never passed both houses, the controversy over slavery in the Mexican cession persisted, dominating congressional attention until after the Civil War.

Lesson Summary

Trouble had been brewing in the American Southwest ever since an indebted Mexican republic attracted immigrants from the U.S. to settle in Texas. Government instability led Texas to declare its independence and petition for annexation into the United States. After admitting the territory, President Polk sent a diplomat to settle old disputes and offer to buy even more land. Mexico refused to discuss anything. Frustrated, Polk sent the army to occupy disputed borderland, leading to the Mexican-American War. From the outset, the war was controversial in the government and with the American people. Trying to settle the slave issue, the Wilmot Proviso suggested banning African-Americans completely from the land, but it failed. Later, popular sovereignty was introduced. California was captured, and then Mexico City fell. The Treaty of Guadalupe Hidalgo largely favored the United States.