"Manifest Destiny was a slogan popular in the 1840s. It was used by people who claimed it was God's will for the U.S. to expand all the way to the Pacific Ocean. These people did not include many Mexicans." —Daria Morgendorffer
The year is 1846. The Southern United States' plantation economy is becoming less and less practical as Europe gets more of its cotton from other sources, particularly India and to a lesser extent Egypt. Industry is driving more and more people north, and the balance between free and slave states is rapidly shifting. This only increases when the Oregon Territory is annexed. The answer: build a railroad from New Orleans to the Pacific coast, where many American expatriates already live. But the land in between is in the hands of a hostile Mexican government...
Meanwhile, European powers are developing spheres of influence in China, and the US government finds itself in need of a large Pacific port, and there's a big one in the Mexican province of Alta California, which everyone knows is much more valuable than the newly-annexed Texas. Unfortunately, relations with the British Empire are tense and there's a great fear that this will be their next conquest.
The answer? War, of course!
Lasting from 1846-1848, the Mexican American War was fought between the United States and Mexico over a small land dispute in Texas. Despite its small origins, it eventually resulted in the invasion of Mexico. Though the war is seldom depicted in contemporary media, both the United States and Mexico as we know them exist, in part, as a result of this war.
Badass Pacifist: Henry David Thoreau refused to pay taxes in protest of this war, which led to his imprisonment (he was bailed out by his aunt much to his frustration). This would lead him to develop his belief in and practice of Civil Disobedience, which became the template for later movement leaders such as Gandhi and the Reverend Martin Luther King.
On the American side, there was Saint Patrick's Battalion, a majority-Catholic group of soldiers who defected from the US Army to side with Mexiconote They were made up generally from immigrants to America who experienced constant racism and mistreatment by the US Army.
Also, several Mexican leaders defected to the US, primarily due to their disgust at Santa Anna's dictatorship and the poor logistics due to corruption.
Divided States of America: California was briefly a separate country early in the war. Arguably, this is Divided States of Mexico, but...
Also, the permissibility of slavery in the territories acquired from Mexico were one of a cluster of major causes of the actual case of Divided States, The American Civil War.
The Empire: Neither sides called themselves empires, but they certainly shared many traits of empires, especially Mexico, which had briefly been an empire, 1821-1823, just after its independence. It became an empire again between 1864-1867.
Expanded States of America: The United States ended up annexing half of Mexico, though that half it was mostly uninhabited at the time.
There's not a lot of Mexicans who like how it ended. Bringing up this war is one of the easiest sources of Misplaced Nationalism in Mexico today.
A few Americans didn't like it either, as it led into The American Civil War. On one side, Abraham Lincoln and other anti-slavery ideologues saw the war as an attempt to expand slavery, what with most of its biggest proponents being from the Southern states.note Amusingly, none of the states admitted from the former Alta California became slave states. Slaveowners did attempt to bring slaves into California and what would become Arizona and New Mexico, but California was soon after admitted as a free state, while the climate in the New Mexico Territory was unsuitable for slave-oriented plantation agriculture. Nevertheless, many people in the southern part of the New Mexico Territory sympathized with the Confederacy during the Civil War, and attempted to secede as the Arizona Territory, with little success. On the other, many Americans expected the annexation of far more territory from Mexico (if not the whole thing), not just the thinly-populated north, even though such a conquest would've been far more than America could bite off, let alone chew, at the time.
Also Robert E. Lee, Winfield Scott, George Meade, and dozens of other Generals and high-ranking officers of The American Civil War. Those who served as Lieutenants and Captains in Mexico were called up for their experience to fight fifteen years later as Generals in their own back yards of Virginia, Tennessee, Mississippi, Alabama, Georgia, Maryland, Pennsylvania, Missouri...
Abraham Lincoln was with the group of congressional Whigs who opposed the war. His record on his anti-slavery position during the war helped win the Republican nomination in 1860 for the Presidency.
Humiliation Conga: The war was a culmination of the Humiliation Conga that had been going on with events such as the Pastry War and the Texas Revolution. The Mexicans suffered military defeat after defeat, most of them routs (except for The Battle of Monterrey, which the Americans still won). The fighting ended with the victorious Americans storming Mexico City. And once the fighting was over, Mexico was forced to sign over half its land (including California), some of which was prime (though as-yet unused) real estate (especially the parts with gold).
The war also left Mexico indebted to European powers which led to France's attempt in the 1860s to invade as part of Napoleon III's plan for empire. When the Mexican Republic fought back and crushed the fledgling empire, that was the end of the humiliation.
Jerkass: Antonio Lopez de Santa Anna. Exiled after his defeat at the hands of the Republic of Texas, Santa Anna offered the Mexican government his military 'experience'. Needing generals, President Farias accepted the offer... not knowing that Santa Anna had been secretly dealing with the United States to quickly end the war and give all contested land to the US. Once in charge of the army, he reneged on both deals by proclaiming himself President again and refusing to deal any further with the United States. He proceeded to mismanage the Mexican forces into a humiliating defeat.
Also qualifies him for Butt Monkey status, as Santa Anna is routinely listed by military historians as one of the worst generals ever.
Not to mention that the In-fighting between the Conservatives and Liberals made fighting much more inefficient in Mexico
Laser-Guided Karma: By adding so much western territory with this war's victory, especially lands south of the Missouri line that limited slavery's expansion, it heated up the slavery debate between North and South and became a prelude to the The American Civil War.
The swift resolution of the Mexican War convinced American military leaders in 1860-61 that the Civil War would be quick and relatively bloodless. Four years and 600,000 casualties later...
Nice Job Breaking It, Hero: When Zachary Taylor (Whig) won early victories that made him an attractive future Presidential candidate, the Democrat President James K. Polk held him back by sending Winfield Scott (another Whig) to finish off the war in Mexico. It didn't work: Taylor won the Presidency as a Whig after a primary against Scott in 1848. And Scott's war heroism made him invaluable as a Union leader at the start of the Civil War (it was Scott's Anaconda Plan that helped win the war, though he was ridiculed for it at the time because few believed, or wanted to believe, Scott's prediction that the Civil War would take years to win).
Starting a war to define the Texas border to the Rio Grande, President Polk? You end up with expanding the United States with a massive land grab that exacerbates the slavery debate into a bloody civil war within fifteen years. Yeah, good plan...
President Polk's Reply: Screw you all, I did what I said I would, no more and no less, and then died. You guys screwed up by electing Franklin Pierce and James Buchanan.
Nice Hat: Those big, feathered American officer's hats.
One Steve Limit: Bizarrly inverted with General Adrian Woll on the Mexican side and General John Wool with the Americans.
Polk is in fact to this day the ONLY U.S. President to keep every one of his campaign pledges (admittedly, there were only four or five).
President Buffoon: Antonio Lopez de Santa Anna, who managed to lose the Southwest, the part of Texas up to the Rio Grande, and (in an unrelated case) Guatemala during his presidency.
The Plan: Though there is little evidence to support this, it is believed that the war was begun by the Americans so they could take California before the British could.
The war occurred as the United States was arguing with the British (in control of Canada) and the Russians over the northern border of the Oregon Territories. By comparison, Polk negotiated a peaceful resolution on the Canadian border, which upset those who wanted to fight and get all of what is now British Columbia. And it especially angered the abolitionists, who saw it as a double standard that he fought hard to gain territory that would be open to slavery, but was quick to compromise on territory that would've been closed to slavery.
Sequel: In many ways, to the Texas Revolution, which occurred in the previous decade, and featured some of the same players, particularly Santa Anna. Also, when this war broke out, the governments of Mexico and the United States still had not agreed exactly on what the borders of Texas werenote A bit of an ongoing issue for Texas going back to the days of the Republic, when their claimed territory included New Mexico and parts of modern-day Mexico and Oklahoma..
Pretext for War: Polk sent an expedition from Texas, which Mexico still claimed, across the Nueces River into an area that both Mexico and Texas had claimed (the area between the Nueces and Rio Grande). When Mexican soldiers shot at Americans for "invading their country", Polk claimed that American blood had been shed on American soil, and got his war.
We ARE Struggling Together: The vicious political infighting in Mexico and unwillingness to fight from the Mexican army were amongst the causes of their defeat. There was also some political divisions in the United States, but the superior training that the U.S. army had made up for it.note The Mexican Army's training methods were notoriously poor [[What an Idiot (to the point they weren't even taught marksmanship), and they also had obsolete weaponry.
In the Dear America series, Valley of the Moon, is the diary of Maria Rosalia de Milagros, who lives in Alta California in 1847. The novel primarily deals with Maria's personal life, but it also has bits about the early parts of the war when California briefly becomes a country.
It must count for something that Zorro lives in California in the era before this war, i.e. California when it was under Spanish/Mexican rule. The Mask of Zorro is actually set on the eve of the Mexican-American War, while its sequel, The Legend of Zorro, picks up shortly after the war and is set on the backdrop of California becoming a state.
Similar to the Zorro example, The Lone Ranger takes place in the brief interlude of Texas statehood prior to the war.