The Mexican-American War | AP US History | Khan Academy

This is a painting of U.S. General Winfield Scott entering Mexico City on September 15, 1847. Scott landed with a U.S. naval fleet several weeks beforehand. He bombarded the coastal stronghold of Veracruz and then fought his way inland toward the capital. Scott actually followed the same route that Spanish conquistador Hernan Cortez took more than 300 years earlier.

Winfield Scott's campaign to Mexico City was just one of three fronts in the two-year-long continent-spanning effort of the United States to take Mexican territory by force. The other two fronts were in California and New Mexico. After the two nations made peace by signing the Treaty of Guadalupe Hidalgo in mid-1848, the United States gained over a million square miles of new territory, a landmass larger than the Louisiana Purchase. For Mexico, this war was a catastrophic defeat which resulted in the loss of about one-third of its total area.

The Mexican-American War doesn't really loom large in American memory compared to the Revolutionary War or the Civil War, but it was a transformative event in the history of the United States and North America. On the scale of national politics, the war led to political realignment and eventually the Civil War. But on a human scale, it led to transformations in the lives of people who lived in the West, who went to bed one day in Mexico and woke up the next day in the United States. National boundaries shifted under their feet. For those people, the outcome of the war meant new laws, customs, new friends and enemies, and even the loss of rights and privileges.

So let's dive a little deeper into the causes and effects of the Mexican-American War. The war began in April of 1846 when a Mexican cavalry brigade attacked U.S. forces who were under the command of General Zachary Taylor across the Rio Grande River from the town of Matamoros, Mexico. After this attack, President James K. Polk sent a war message to Congress. He fumed that the Mexican troops had invaded our territory and shed American blood on American soil.

Now back up a minute. You may be wondering, as many keen observers did at the time, what exactly were U.S. forces doing there near the Rio Grande River in the first place? The answer to that reveals the two major causes of the war: Texas annexation and Manifest Destiny. Let's start by talking about Texas annexation. American settlers, many of whom were slave owners, had been moving to Texas since the 1820s, when the region was still controlled by Spain. After Mexican independence, the country outlawed slavery, but the American settlers resisted the Mexican government's authority.

In 1836, they rebelled and won independence for Texas. They requested that the United States annex the new nation shortly thereafter, but adding another slave state to the union was politically dangerous for the administration at that time. So Texas remained an independent nation until 1845. In 1845, Democratic President James K. Polk took office. Now Polk was an ardent expansionist. He was a believer in Manifest Destiny, this idea that God wanted the United States to expand across the North American continent.

Polk wanted to annex Texas, which his administration undertook immediately. He also desperately wanted California, which was a hub of commerce on the Pacific Ocean. This is actually before gold was discovered there. So Polk sent a representative to the Mexican government offering to buy California, but Mexico said California was not for sale. Polk was determined to get this territory with blood or money, so he came up with an alternate plan.

The border between Mexico and Texas was under dispute, so Polk directed General Zachary Taylor to go down into this disputed territory and provoke hostilities. And that's exactly what happened when the Mexican cavalry attacked Taylor's forces. As far as Mexico was concerned, Taylor's troops were invading their country, and they had no choice but to defend it. Despite Polk's war message saying that American blood had been shed on American soil, many U.S. politicians were also skeptical about who started the war and where.

A young Whig congressman from Illinois named Abraham Lincoln demanded that Polk show him the exact spot where American blood had been shed. The war that ensued was longer, costlier, and deadlier than the U.S. government had estimated, which is often the case with wars. At its conclusion, Polk had achieved his vision for Manifest Destiny in the Treaty of Guadalupe Hidalgo, which ended the war.

The United States agreed to pay Mexico 15 million dollars, and in exchange, Mexico ceded Texas, California, and most of the modern-day Southwest to the United States. So what were the effects of this war? Well, the addition of this Mexican cession territory had far-reaching consequences for both the United States and the residents of the West. The existing residents of the territory, including Mexicans, Native Americans, and the descendants of Spanish colonists, found that life under the rule of the United States could be very different than under the rule of Mexico.

Where Mexican law had abolished slavery and prescribed equality under the law for people regardless of color, the Texas Constitution permitted slavery and denied civil rights to non-white residents. For other residents of the territory, life didn't change much at all. Huge swathes of the West were actually controlled by Native American nations like the Comanche Empire, which didn't care whether the distant government who claimed their territory on paper was located in Mexico City or in Washington, D.C.

For the United States government, the addition of this new territory was political kryptonite. Both Northerners and Southerners were convinced that the opposite region was conspiring to limit their economic opportunities in the West. During the war, Congressman David Wilmot of Pennsylvania introduced a resolution in the House that would prohibit slavery in any territory gained from the conflict. The reaction to the Wilmot Proviso showed just how big the sectional divide in the country was becoming since party lines broke down entirely.

Northerners, Whig and Democrat alike, voted for the Wilmot Proviso, and Southerners, Whig and Democrat alike, voted against it. Ultimately, the Proviso passed in the House but was defeated in the Senate. And then, gold was discovered in California just before the end of the war, sending hordes of prospectors west and making statehood for California an urgent issue that would soon upset the balance of power between free and slave states in Congress.

In other words, we can draw a direct line from the Mexican War to the breakdown of the second party system, which was replaced by a solidly Southern Democratic Party and a new Northern Republican Party, and from there to the Civil War.