The late 1840s were a period of ardent American expansionism. The annexation of Texas, the election of James K. Polk, and the incorporation of the Oregon Territory all promoted a feeling of vibrant nationalism, a spirit of opportunity and progress that spread across the American continent. The industrial revolution was beginning in the Northeast, the standard of living was rising for most whites, and chances for self-improvement and land acquisition for ordinary white Americans seemed to be increasing every day. Americans lived longer in more wide open spaces and produced larger families than their European counterparts. They embarked upon new frontiers in the West, where vast tracts of land appeared to be ripe for the taking, crawling with abundant game for hunting and rich soil for planting. Of course, hundreds of thousands of Native Americans had inhabited regions like the Great Plains, the Southwest, and the Pacific Coast for centuries, but their nomadic lifestyle enabled settlers to continue the ongoing process of encroachment. In the eyes of the whites, the Indians' migratory social structure also made them unfit to "properly" settle the land according to the Western model of agricultural production.
Racial beliefs that upheld the Anglo-Saxon as the most advanced, civilized, and righteous of all the races undergirded a popular willingness to go to war with Mexico over Texas. Yet even racist white Americans in the free states did not like the idea of watching slavery expand westward along with the national boundary line. Since 1802, Congress had maintained an unspoken but unbroken rule of admitting free and slave states in tandem in order to maintain an equilibrium of sectional representation in the Senate. With the addition of more than half a million square miles of territory from a defeated Mexico, the vexing issue of slavery's expansion into the new lands soon became the central concern of American politics.
At the outset of the Mexican American War, strong national parties traversed geographic sections on the basis of mutual interest, with Democrats and Whigs both running strong in both North and South. But during the period of rapid expansion epitomized by James Polk's presidency (1845-49), sectional divisions in Congress began to come to the forefront. Northerners of every party supported Congressman David Wilmot's Proviso that attempted, unsuccessfully, to prevent slavery from spreading into any of the new territories; southerners of every party opposed it. For decades, the slavery issue had been quieted by Constitutional compromises and the cross-sectional alliance of interests embodied in the two-party system. During the heyday of Manifest Destiny, those compromises and cross-sectional alliances began to collapse. One signal of slavery's growing prominence as a political hot button was the House of Representatives' abandonment of its longstanding gag rule, which had helped congressmen agree to disagree about slavery by banning them from discussing it.
It was not because of a sudden change of heart on the part of politicians (or white Americans in general) that slavery reemerged as a pivotal national issue. Sympathy for enslaved black people did exist within certain circles, particularly among the evangelical Christians of the Northeast and Old Northwest. Yet abolitionism remained a small movement on the radical fringe of American politics. Northern opposition to the extension of slavery into new western lands grew less out of sympathy for enslaved blacks than out of fears for free whites. For northern white men from the emerging working class, who composed the bulk of voters in the region, slavery posed a threat to their livelihoods and an insult to their status as free workers. They did not want to live alongside blacks, nor did they want to be subjected to competing with slave labor. They envisioned the new western territories as the land of opportunity for free white families to achieve independence and prosperity, and they were dead set against slave labor displacing free labor there. As avid proponents of democracy for white men, these voters resented the "Slave Power," the quasi-aristocratic encroachment of slaveowning southerners who wielded disproportionate influence in Congress. During Polk's presidency, northerners complained, the Slave Power sold out legitimate American claims to half the Oregon Territory (future free states) even as it drummed up war against Mexico under phony pretenses in order to seize northern Mexican provinces (future slave states). To white northerners, the expansionistic Slave Power threatened to undermine everything they stood for and believed in, from the free labor system to the American dream of success and prosperity through hard work, to the democratic system and the balance of power itself.
Meanwhile, southerners increasingly perceived themselves as a persecuted minority. They viewed their situation with growing alarm, for they feared losing parity in the Senate, which would allow the northern majority to outlaw their "peculiar institution" of slavery. Since the 1830s, radical abolitionists had deluged the South with inflammatory pamphlets, complete with illustrations of the worst abuses of slave life. For a white southern culture centered on manners and reputation, this was a direct insult and an outrage. Yet such attacks had been waged by only a small subset of the northern population; now northerners of both parties were uniting against the possible spread of slavery into western lands.
The "free soil, free labor, free men" platform embodied the Free Soil Party of 1848, which played a central role in depriving Polk's Democrats of another presidential term, and which ultimately formed a critical base for the Republican Party that emerged six years later and elected Abrham Lincoln to the White House on an antislavery platform in 1860.
Manifest Destiny drove the military conquest of western lands, but its faith in the virtues and strengths of the white race masked a critical divide within that population. In retrospect, it is clear that a chain of events that began with James Polk's election and the Mexican-American War ended in irreconcilable sectional conflict and Civil War. Yet the potency of Manifest Destiny persisted long after the Civil War had concluded; its quasi-religious self-certainty and expansionistic, patriotic spirit continued to inspire presidential addresses, imperial wars, and international interventions for generations to come.