This section of American history is part of what is known as the antebellum (before war) era. The period begins with territorial expansion and the great migration of pioneers across the continent west of the Mississippi. The idea of manifest destiny—that the United States was bound sooner or later to occupy the entire North American Hemisphere—came of age in the 1840s. It began with the annexation of Texas, which led to the Mexican-American War and the great land cession that resulted from it. It was also the time of “Oregon Fever” as wagon trains took pioneers over the Rocky Mountains to the Willamette and Columbia River valleys.

The addition of these new territories, which eventually became new states, reopened the debate over the expansion of slavery, which had been more or less settled by the Missouri Compromise of 1820. That debate dominated the decade of the 1850s as the nation drifted inexorably toward separation and the Civil War, sometimes called the “War Between the States,” or the “Second American Revolution.”

Those years were also marked by significant social changes resulting from a number of factors. The discovery of gold in California spurred a huge migration across the continent (or around Cape Horn) as well as immigration from Central and South America and Asia. A new wave of immigration from Europe centered around immigrants from Ireland, attracted to the United States because of opportunities unavailable at home, spurred in the 1840s by the potato famine that devastated the Irish homeland. The year 1847, sometimes referred to as "Black 47" was the worst of the famine years and is remembered to this day among people of Irish descent.

The other events of the period, religious revival, the temperance movement, prison and insane asylum reform, reform and innovation in education and the beginning of a new era in American literature often found expression in matters relating to the new states and territories. Those movements will be covered in detail in the next section on Antebellum America.

The most important developments relative to the expanding nation were those relating to slavery. The abolition movement in the North was in full force. The British Empire had finally extinguished slavery in all its possessions and was attacking the continuing international slave trade. Religious groups saw slavery as unchristian, though not in Southern congregations.

Thus after a period of relative calm following the Missouri Compromise, more and more Americans noted with considerable discomfort that the slavery issue was not only still present, but becoming ever more heated. Gag orders muzzled Congress, personal liberty laws challenged federal authority over fugitive slaves, abolitionists thundered, and Southern plantation owners began calling slavery a “positive good,” a notion probably unthinkable before 1800. By 1850 talk of secession and war was common both inside and outside Congress, and every election from 1844 through 1860 had the “peculiar institution” as a backdrop. It was only a matter of time until something blew up.

Tension continued to rise throughout the decade of the 1850s, starting with passage of the 1850 Compromise laws. They were followed by the publication of Uncle Tom's Cabin, which in turn was followed by violent outbreaks in Northern cities as slave trackers captured runaways and took them back to the South. Passage of the Kansas-Nebraska Act in 1854 led to violence in the Kansas Territory, and the Dred Scott decision both clarified and exacerbated the slavery issue. Lincoln's famous debates with Senator Stephen Douglas during the 1858 Senate race in Illinois, and John Brown's raid on Harper's Ferry and subsequent trial and death by hanging, kept the issue burning. Finally, the divisive election of 1860 led to the secession of South Carolina. Ten additional states eventually joined South Carolina to form the Confederate States of America.

The second generation of American leadership was dying—by 1852 Calhoun, Clay and Webster were all gone. The party system took firmer hold as the two major parties—the Democrats and Whigs—vied for the presidency and 1840, 1848 and 1852. The Whig Party broke up following the election of 1852, partly because of disagreements of slavery, but then another party arose: The Republican Party was founded in Ripon, Wisconsin, in 1854. Democrat Stephen Arnold Douglas of Illinois became the most powerful public figure of the early 1850s, and Jefferson Davis gradually assumed the mantle of John Calhoun as spokesman for the South. Although Abraham Lincoln had served in Congress from 1847 to 1849, he was all but unknown outside Illinois, even while pursuing higher office, always unsuccessfully. He arrived as a national figure in 1858 on the strength of his debates with Douglas, although he lost that race. But his eloquent speeches, widely published, brought him to the presidency in 1861.

The Civil War is the central event of the American past. In a real sense it divides American history into two major segments: before and after that great conflict, which was also the deadliest in American history. It is generally understood that the Civil War brought the end of slavery, but it did much more. The war between the states changed the relationship between the federal government and the states forever and provided a backdrop for the beginning of the civil rights movement for African-Americans, a movement which is still being played out. (The election of Barack Obama in 2008 is the most significant recent milestone in that movement.)

Early American history has a plot; it has a beginning, a middle, and an end. The beginning was the period of colonization and colonial development when Europeans flocked to America’s shores and built a new society. The middle section of the story was the era of the American Revolution, the creation of a new and democratic republic unlike anything the world had seen before. But in the latter decades of that long middle period of American political, economic, cultural, and social growth, the beginning of the end of the early part of American history began to loom, as secession and the possibility of war welled up across the American landscape. By the end of this period of American history the nation stretched from coast to coast. The states of California and Oregon had joined the Union, and the territory which eventually became the “lower 48”—the area between Mexico and Canada—was intact. Although much of the western territory remained sparsely populated, the outlines of the trans-continental nation had been formed.