of US general Winfield Scott

entering Mexico City on September 15th, 1847.

Scott landed with a US 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 Cortes 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 1/3 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.

A Mexican cavalry brigade attacked US 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 US forces doing there

near Rio Grande River in the first place?

And 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 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.

Now 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 US 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 US 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,

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 resident 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 proscribed 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 DC.

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 Priviso

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

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.