Origins of the American Civil War: "Border Ruffians" from Missouri invade Kansas and force election of a pro-slavery legislature.
Border ruffians were proslavery raiders, crossing from the slave state of Missouri into the Kansas Territory, to help ensure Kansas entered the Union as a slave state. They were a key part of the violent period called Bleeding Kansas, that peaked from 1854 to 1858. Their crimes included fraudulent voting (falsely saying they were residents of Kansas), interference with elections, and raiding, intimidating, and destroying property of "Free-State" (anti-slavery) settlers. Some took pride in their criminal reputation. Many became pro-Confederate guerrillas, or bushwhackers.
Historians who debate the origins of the American Civil War focus on the reasons that seven Southern states (followed by four other states after the onset of the war) declared their secession from the United States (the Union) and united to form the Confederate States (simply known as the "Confederacy"), and the reasons that the North refused to let them go. Proponents of the pseudo-historical Lost Cause ideology have denied that slavery was the principal cause of the secession. While historians in the 21st century agree on the centrality of the conflict over slavery—it was not just "a cause" of the war but "the cause"—they disagree sharply on which aspects of this conflict (ideological, economic, political, or social) were most important.The principal political battle leading to Southern secession was over whether slavery would be permitted to expand into newly acquired 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 population. 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.Abraham Lincoln won the 1860 presidential election, even though he had not been on the ballot in ten Southern states. His victory triggered declarations of secession by seven slave states of the Deep South, all of whose riverfront or coastal economies were based on cotton that was cultivated by enslaved labor. They formed the Confederate States after Lincoln was elected in November 1860 but before he took office in March 1861.
Nationalists in the North and "Unionists" in the South refused to accept the declarations of secession. No foreign government ever recognized the Confederacy. The U.S. government, under President James Buchanan, refused to relinquish its forts that were in territory claimed by the Confederacy. The war itself began on April 12, 1861, when Confederate forces bombarded Fort Sumter, a major fortress in the harbor of Charleston, South Carolina.
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." Pulitzer Prize-winning author David 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 had fellowship with slaveholders, to be preserved. Thus they were committed to values that could not logically be reconciled."Other important factors were partisan politics, abolitionism, nullification versus secession, Southern and Northern nationalism, expansionism, economics, and modernization in the Antebellum period.