American Civil War · CHAPTER 1 ·
Mexican–American War
Chapter 1 of The American Civil War, 1840–1877: “Manifest Destiny and the Mexican War”
In the 1840s the belief in Manifest Destiny drove American expansion: Texas joined the United States in 1845 and Oregon was split with Britain in 1846. When Mexico refused to sell California, President Polk sent troops into disputed land, and the Wilmot Proviso of 1846 tied the conquest to the question of slavery.
WW2 is free — every chapter. Try chapters 1–2 of any 2 other atlases (3 with a free account). Opening a new atlas uses one of them. Membership opens all 21 — Join.
Free printable American Civil War study guide, sent by email ↓
The map above is the world of North America, July 1848 — The Cession — and the question.
Part of IB History: The American Civil War →
BRIEF EXPLANATION — THE ESSENTIAL ARGUMENT
The question this atlas follows from first chapter to last is how the United States came to fight a civil war over slavery, and the road begins with a war of conquest that most Americans expected to make their country stronger. In the 1840s many believed in what an 1845 magazine editorial called their “manifest destiny to overspread the continent,” a conviction that the republic had a God-given right to expand, much as some nations today speak of a historic mission. Texas, a slaveholding republic since it broke from Mexico in 1836, joined the United States in 1845, although Mexico had warned that annexation would mean war. When Mexico refused to sell California, President James K. Polk sent troops into land both countries claimed and got his war. For Mexico, already weakened by coups, debt and raids by the Comanche, a mounted nation of the southern plains, the war was an invasion that cost about half its territory, a loss its divided government was too weak to prevent.
Victory created a problem the country could not digest, because every acre won had to be organised as slave or free. In 1846 Congressman David Wilmot proposed banning slavery from any land taken from Mexico, and the House divided by section, Northerners of both parties for the ban and Southerners against it. That vote showed that slavery could now split the Democrats and the Whigs, national parties that needed votes in North and South alike, like modern parties spanning very different regions. On the map the conquered land appears in a blue-and-white hatch, owned yet undecided. The Compromise of 1850, a package of laws in which each side won something, much like a modern bipartisan deal, admitted California as a free state and left Utah and New Mexico to decide for themselves. It bought peace by postponing the one decision that mattered, whether Congress could keep slavery out of new land.
The full story
Expansion in the 1840s was a political programme as much as a migration, and President James K. Polk pursued it to the point of war. Its champions called it Manifest Destiny, the belief that Providence, meaning God’s plan, intended the United States to span the continent, much as some nations today claim a historic mission, and it carried a racial confidence that white Americans should rule. Texas, a slaveholding republic since it broke from Mexico in 1836, joined in 1845 by joint resolution, a simple vote of both houses like an ordinary law, after the Senate rejected an annexation treaty. Oregon was split with Britain at the 49th parallel, today’s Canadian border, and its map hatching vanishes in June 1846. When Mexico refused to sell California, Polk sent General Zachary Taylor into the hatched strip between the Nueces River and the Rio Grande, and a skirmish there in April 1846 let him present a war of choice as a war of defence.
Mexico experienced the same events as an invasion meant to dismember the country, which Mexicans still call the American intervention, and it lost as much to its own crisis as to American strength. The young republic was nearly bankrupt, and its politics swung between federalists, who wanted strong states, and centralists, who wanted power in Mexico City, a quarrel much like the American argument over states’ rights. Presidents changed more than half a dozen times in 1846–48, and none could sell land and survive. Raids by the Comanche and Kiowa, mounted nations of the southern plains, had already killed thousands in the far north. Mexicans fought hard all the same. Santa Anna, Mexico’s leading general, marched north to Buena Vista along the dark red arrow, and cadets died defending Chapultepec, the castle guarding the capital, as the American general Winfield Scott fought inland from Veracruz. A state that could not agree on who governed it could not hold a frontier it had never fully controlled.
Victory turned expansion into the slavery question, because every territory, land Congress governed — as it still governs Guam — until it had settlers enough to become a state, had to be opened or closed to slavery. In 1846 Congressman David Wilmot attached a proviso to a bill giving Polk $2 million to negotiate peace, a condition of the kind lawmakers still attach to budget bills, barring slavery from any land taken from Mexico. The House passed it on a North–South vote and the Senate blocked it, so the question stayed open. Gold found in 1848 filled California so fast that it sought statehood as a free state, threatening the even split of slave and free states that gave the South an equal voice in the Senate. The Compromise of 1850, a package of trade-offs like a modern bipartisan deal, admitted California, left Utah and New Mexico to popular sovereignty, a local referendum of their settlers, and gave the South a Fugitive Slave Act forcing Northern officials to help return people who had escaped. The hatching that survives on the 1850 map marks the question it postponed.
DEEP EXPLANATION — EVIDENCE & INTERPRETATION
What “Manifest Destiny” meant, and who doubted it
The belief in a continental destiny was popular and contested at once, and historians still argue about what drove it. An unsigned 1845 editorial, usually credited to the editor John L. O’Sullivan, named it “our manifest destiny to overspread the continent allotted by Providence.” Reginald Horsman traced a racial strand, an Anglo-Saxonism that cast Mexicans and Native peoples as unfit to govern their own land. Thomas Hietala read the same speeches as anxious, arguing that fear of British power and of markets glutted with unsold goods pushed expansion forward. Slavery’s role is the sharpest debate. Abolitionists, who wanted slavery ended, saw a plot by the “Slave Power,” a supposed conspiracy of slaveholders running Washington, and the evidence for Texas supports them, since Secretary of State John C. Calhoun defended annexation in 1844 as a shield for slavery, while Polk’s hunger for California was about harbours and trade. Opposition to the war was loud and included future leaders of the Union. Lincoln’s “spot resolutions” of 1847 challenged Polk’s account of how the war began, and Grant later judged it “one of the most unjust ever waged by a stronger against a weaker nation.” Racial ideas limited conquest as well as driving it, since the 1848 campaign to annex all of Mexico foundered partly on an unwillingness to absorb millions of Mexicans.
Mexico’s war: raids, coups and a divided state
Historians on both sides of the border now explain the war through Mexico’s internal crisis as well as Washington’s ambitions. Brian DeLay has shown that Comanche and Kiowa raids from the 1830s to 1847 struck ten northern Mexican states, killing thousands and deepening the north’s grievances against Mexico City, so the US Army met a frontier already exhausted. Politics in the capital compounded the damage, because every faction feared being seen to bargain away land. General Mariano Paredes overthrew President José Joaquín Herrera at the turn of 1846 partly because Herrera seemed willing to receive Polk’s envoy, and Santa Anna, let through the US blockade on a private promise of peace, took command and fought. When Scott landed in March 1847, the capital had just weathered the revolt of the polkos, militia units rising against a decree seizing church property, and Yucatán had broken away, so Mexico met the invasion divided. Losses are uncertain because records were incomplete: perhaps 5,000 Mexican soldiers died in battle, and with disease and civilian deaths the toll may have reached 25,000. Mexican writers in 1848 blamed American greed, while Josefina Zoraida Vázquez weighs that greed against a state not yet consolidated. Recent scholarship therefore asks why a large nation could not defend itself, and the answer runs through Mexico City as well as Washington.
Why 1850 was an armistice
The Compromise of 1850 is often remembered as a grand settlement, yet the votes behind it reveal something fragile. Henry Clay bundled the measures into one omnibus bill, a single package of many laws such as Congress still uses for budgets, and the Senate tore it apart on 31 July 1850. Stephen A. Douglas then passed the pieces one at a time, and Holman Hamilton’s study of the roll calls showed that each was carried by a different coalition. Northerners supplied the votes for the free-state measures, Southerners for those that favoured slavery, and only a small bloc of Northern Democrats and men from the border slave states voted for nearly everything, so no majority ever endorsed the package as a whole. The death of President Zachary Taylor on 9 July 1850 helped, because Taylor had threatened a veto and his successor, Millard Fillmore, signed every bill. David Potter judged the result an armistice, a pause in the conflict that settled no principle. Popular sovereignty was its weakest joint, because Northern and Southern Democrats disagreed about whether settlers could decide during the territorial stage or only at statehood. A settlement each section accepted only in the parts it liked could last only until someone tested those parts.
Key places on the map
On this day
Why it happened
Land hunger and an ideology of destiny. Americans were moving west faster than their government could follow, so Washington came under pressure to secure land its citizens had already entered. Thousands took the Oregon Trail in the 1840s, the brown arrow on the map. Expansionists dressed this movement in the language of Manifest Destiny, the belief that Providence had given the continent to the United States. The idea joined real hopes, such as Pacific ports for the China trade, to a racial conviction that Mexicans and Native peoples should give way.
Polk’s determination to have California. The most direct cause was a president who wanted California badly enough to take it. Polk entered office in 1845 with a short list of goals, and California, with its harbours at San Francisco and San Diego, stood near the top. He first tried to buy it, sending John Slidell to Mexico City authorised to offer up to $25 million by most accounts, and the Mexican government refused to receive him. Polk then moved Taylor’s army to the Rio Grande, into land that Mexico held and Texas claimed, which turned a diplomatic refusal into an armed standoff.
A Mexican state at its weakest. Mexico in 1846 was in no condition to fight a long war. Its treasury was empty, its army was led by generals who also made and unmade governments, and its northern states had been devastated by Comanche and Kiowa raids. Mexican leaders could not sell territory without being overthrown, as Herrera discovered when Paredes toppled him for being willing to talk. Weakness made war more likely, because it left Mexico able neither to deter Polk nor to bargain with him.
Texas and the politics of slavery. Texas joined the Union as a slave state, and its annexation was argued in the language of slavery from the start. In 1844 Secretary of State Calhoun told the British minister that annexation would protect slavery from British abolitionism, and the Senate rejected the treaty that June partly for that reason. When annexation passed by joint resolution in 1845, Northern opponents saw the “slave power,” the slaveholders’ supposed grip on the federal government, enlarging itself. The war inherited that suspicion, which is why Northern congressmen moved at once to keep slavery out of its conquests.
THE TURNING POINT
The Wilmot Proviso, 8 August 1846. The hinge of the decade came on the night a first-term Pennsylvania congressman added one sentence to a bill giving Polk $2 million to make peace. David Wilmot proposed that slavery should never exist in any territory taken from Mexico, and the House approved it 83 to 64, with Northern Democrats and Whigs together on one side and Southerners of both parties on the other. That vote revealed that slavery could now split the national parties, which had held the Union together by making Northerners and Southerners cooperate. The Senate never passed it, yet its question shaped every crisis that followed, in 1850, 1854 and 1860. An amendment that failed did more to divide the country than most laws that passed.
What it changed
Free soil becomes a party. The proviso gave Northerners who opposed slavery’s spread a cause, free soil, meaning western land kept for free labour, that did not require them to attack slavery where it already existed. In 1848 the Free Soil Party ran former president Martin Van Buren on that platform and won about ten percent of the popular vote. Its slogan, “Free Soil, Free Speech, Free Labor, and Free Men,” and many of its leaders passed into the Republican Party in the 1850s. Lincoln’s platform of 1860 was, in effect, the Wilmot Proviso grown up.
The Fugitive Slave Act radicalizes the North. The South’s main gain in 1850 became the North’s main grievance. The new law denied accused fugitives a jury trial and paid federal commissioners $10 when they sent a person south and $5 when they set one free. Crowds in Boston and other cities tried to rescue captured people, and Harriet Beecher Stowe wrote Uncle Tom’s Cabin in response. A law meant to reassure the South taught many Northerners that slavery reached into their own towns.
Popular sovereignty carries the quarrel to Kansas. Letting settlers decide sounded like a democratic way out, and in 1854 Stephen Douglas applied it to Kansas and Nebraska, repealing the line of 36°30′ that the Missouri Compromise of 1820 had drawn between slave and free territory. On the continental map the hatch spreads across the plains in the snapshot of June 1854. Settlers then fought over the decision, and Kansas became a small civil war. The method of 1850 moved the conflict from Congress to the frontier, where no one could arbitrate it.
Mexico trains the generals of 1861. The Mexican War was the school where the commanders of 1861 learned their trade, since Grant, Lee, Jackson, Longstreet, McClellan and Jefferson Davis all served in it. Grant wrote later that the Southern rebellion “was largely the outgrowth of the Mexican war,” and he judged the Civil War the nation’s punishment for that conquest. The war that reopened the slavery question also trained the men who would fight over it, each knowing the others’ habits.
DISCUSSION QUESTION — Was the Mexican War a war for slavery?
The case for yes begins with Texas, which entered as a slave state after Calhoun had defended annexation as protection for slavery, and it gains force from Southern enthusiasm for the war. The case for no rests on Polk’s aims, which centred on California’s harbours, on the Northern Democrats who backed the war, and on the Southern Whigs who opposed it for fear of what new land would do to the Union. Geography weakened the slavery motive too, since few planters expected cotton to grow in most of the Mexican Cession, the land Mexico gave up in 1848. The stronger verdict separates motive from consequence. The war was fought chiefly for land and ports, yet once won it forced the question of slavery’s expansion onto every vote that followed, so slavery decided what the war meant even where it did not decide why it began. The same distinction serves for many wars, whose causes and consequences often turn out to be different things.
An interesting fact
The treaty that ended the war was signed by a man who had been fired. Polk recalled his envoy Nicholas Trist in October 1847, but Trist received the order in mid-November and chose to stay, explaining himself in a letter of some sixty-five pages because he judged that Mexico’s moderate government might fall before a replacement arrived. He signed at Guadalupe Hidalgo on 2 February 1848. Polk, furious, sent the treaty to the Senate anyway, since it gave him almost everything he wanted. Trist was dismissed and left unpaid until 1871, when Congress, pressed by Senator Charles Sumner, finally paid his salary and expenses.
Related
THE FREE PRINTABLE STUDY GUIDE
Join the Dispatch — a short welcome series, then a letter when a new atlas opens — and the printable study guide for The American Civil War is yours now, free.
NO TRACKING · YOUR ADDRESS IS USED FOR THE DISPATCH AND NOTHING ELSE · UNSUBSCRIBE ANYTIME