information:highlighting, quality:corruption
Origins

A consensus of historians who address the origins of the war agree that the preservation of the institution of slavery was the principal aim of the eleven Southern states (seven states before the onset of the war and four states after the onset) that declared their secession from the United States (the Union) and united to form the Confederate States of America (known as the "Confederacy"). However, while historians in the 21st century agree on the centrality of slavery in the conflict, they disagree sharply on which aspects of this conflict (ideological, economic, political, or social) were most important, and on the North 's reasons for refusing to allow the Southern states to secede. Proponents of the pseudo-historical Lost Cause ideology have denied that slavery was the principal cause of the secession, a view that has been disproven by the overwhelming historical evidence against it, notably some of the seceding states' own secession documents.

The principal political battle leading to Southern secession was over whether slavery would be permitted to expand into the Western territories destined to become states. Initially, Congress had admitted new states into the Union in pairs, one slave and one free. This had kept a sectional balance in the Senate but not in the House of Representatives, as free states outstripped slave states in numbers of eligible voters. Thus, at mid-19th century, the free-versus-slave status of the new territories was a critical issue, both for the North, where anti-slavery sentiment had grown, and for the South, where the fear of slavery's abolition had grown. Another factor leading to secession and the formation of the Confederacy was the development of white Southern nationalism in the preceding decades. The primary reason for the North to reject secession was to preserve the Union, a cause based on American nationalism.

Background factors in the run up to the Civil War were partisan politics, abolitionism, nullification versus secession, Southern and Northern nationalism, expansionism, economics, and modernization in the antebellum period. As a panel of historians emphasized in 2011, "while slavery and its various and multifaceted discontents were the primary cause of disunion, it was disunion itself that sparked the war." Historian David M. Potter wrote: "The problem for Americans who, in the age of Lincoln, wanted slaves to be free was not simply that southerners wanted the opposite, but that they themselves cherished a conflicting value: they wanted the Constitution, which protected slavery, to be honored, and the Union, which was a fellowship with slaveholders, to be preserved. Thus they were committed to values that could not logically be reconciled."