"The origins of the Civil War may be traced to a complex mix of causes, some of which can be traced back to the earliest years of American colonization. Principal among the issues were the following:
Slavery
Slavery in the United States first began in Virginia in 1619. By the end of the American Revolution, most northern states had abandoned the institution while it continued to grow and flourish in the plantation economy of the South. In the years prior to the Civil War almost all sectional conflicts revolved around the slave issue. This began with the debates over the three-fifths clause at the Constitutional Convention of 1787 and continued with the Compromise of 1820, the Nullification Crisis, the anti-slavery Gag Rule, and the Compromise of 1850.
Slavery and Control of the Government
Throughout the first half of the 19th century, Southern politicians sought to defend slavery by retaining control of the federal government. While they benefited from most presidents being from the South, they were particularly concerned about retaining a balance of power within in the Senate. As new states were added to the Union, a series of compromises were arrived at to maintain an equal number of "free" and "slave" states.
In 1820, Maine entered as a free state while Missouri joined as a slave state. The balance was finally disrupted in 1850, when Southerners permitted California to enter as a free state in exchange for laws strengthening slavery. This balance was further upset with the additions of free Minnesota (1858) and Oregon (1859).
Two Regions on Separate Paths
The widening of the gap between slave and free states was symbolic of the changes occurring in each region. While the South was devoted to an agrarian plantation economy with a slow growth in population, the North had embraced industrialization, large urban areas, infrastructure growth, as well as was experiencing high birth rates and a large influx of European immigrants. This boost in population doomed Southern efforts to maintain balance in the government as it meant the future addition of more free states and the election of a Northern, potentially anti-slavery, president.
Slavery in the Territories
The political issue that finally moved the nation towards conflict was that of slavery in the western territories won during the Mexican-American War. A similar issue had been dealt with earlier, in 1820, when, as part of the Missouri Compromise, slavery was permitted in the Louisiana Purchase south of 36°30'N latitude (the southern border of Missouri). Rep. David Wilmot attempted to prevent slavery in the new territories in 1846, when he introduced the Wilmot Proviso in Congress. After extensive debate it was defeated.
In 1850, an attempt was made to resolve the issue. A part of Compromise of 1850 called for slavery in the unorganized lands (largely Arizona & New Mexico) received from Mexico to be decided by popular sovereignty. This meant that the local people and their territorial legislatures would decide for themselves whether slavery would be permitted. Many thought that this decision had solved the issue until it was raised again in 1854 with the passage of the Kansas-Nebraska Act.
"Bleeding Kansas"
Proposed by Sen. Stephen Douglas of Illinois, the Kansas-Nebraska Act essentially repealed the line imposed by the Missouri Compromise. Douglas, an ardent believer in grassroots democracy, felt that all the territories should be subject to popular sovereignty. Seen as a concession to the South, the act led to an influx of pro- and anti-slavery forces into Kansas. Operating from rival territorial capitals, the "Free Staters" and "Border Ruffians" engaged in open violence for three years.
Though pro-slavery forces from Missouri had openly and improperly influenced elections in the territory, President James Buchanan accepted their Lecompton Constitution, and offered it to Congress for statehood. This was turned down by Congress which ordered a new election. In 1859, the anti-slavery Wyandotte Constitution was accepted by Congress. The fighting in Kansas further heightened tensions between North and South.
States' Rights
As the South recognized that control of the government was slipping away, it turned to a states' rights argument to protect slavery. Southerners claimed that the federal government was prohibited by the Tenth Amendment from impinging upon the right of slaveholders take their "property" into a new territory. They also stated that the federal government was not permitted to interfere with slavery in those states where it already existed. They felt that this type of strict constructionist interpretation of the Constitution coupled with nullification, or perhaps secession would protect their way of life.
Abolitionism
The issue of slavery was further heightened by the rise of the Abolitionist movement in the 1820s and 1830s. Beginning in the North, adherents believed that slavery was morally wrong rather than simply a social evil. Abolitionists ranged in their beliefs from those who thought that all slaves should be freed immediately (William Lloyd Garrison, Frederick Douglas) to those calling for gradual emancipation (Theodore Weld, Arthur Tappan), to those who simply wanted to stop the spread of slavery and its influence (Abraham Lincoln).
Abolitionists campaigned for the end of the "peculiar institution" and supported anti-slavery causes such as the Free State movement in Kansas. Upon the rise of the Abolitionists, an ideological debate arose with the Southerners regarding the morality of slavery with both sides frequently citing Biblical sources. In 1852, the Abolitionist cause received increased attention following the publication of the anti-slavery novel Uncle Tom's Cabin. Written by Harriet Beecher Stowe, the book aided in turning the public against the Fugitive Slave Act of 1850.
John Brown's Raid
John Brown first made a name for himself during the "Bleeding Kansas" crisis. A fervent abolitionist, Brown, along with his sons, fought with anti-slavery forces and were best known for the "Pottawatomie Massacre" where they killed five pro-slavery farmers. While most abolitionists were pacifists, Brown advocated violence and insurrection to end the evils of slavery.
In October 1859, financed by the extreme wing of the Abolitionist movement, Brown and eighteen men attempted to raid the government armory at Harper's Ferry, VA. Believing that the nation's slaves were ready to rise up, Brown attacked with the goal of obtaining weapons for the insurrection. After initial success, the raiders were cornered in the armory's engine house by local militia. Shortly thereafter, US Marines under Lt. Col. Robert E. Lee arrived and captured Brown. Tried for treason, Brown was hanged that December. Before his death, he predicted that "the crimes of this guilty land will never be purged away; but with Blood."
The Collapse of the Two-Party System
The tensions between North and South were mirrored in a growing schism in the nation's political parties. Following the compromise of 1850 and the crisis in Kansas, the nation's two major parties, the Whigs and Democrats, began to fracture along regional lines. In the North, the Whigs largely blended into a new party: the Republicans.
Formed in 1854, as an anti-slavery party, the Republicans offered a progressive vision for the future that included an emphasis on industrialization, education, and homesteading. Though their presidential candidate, John C. Frémont, was defeated in 1856, the party polled strongly in the North and showed that it was the Northern party of the future. In the South, the Republican Party was viewed as a divisive element and one that could lead to conflict.
Election of 1860
With the division of the Democrats, there was much apprehension as the election 1860 approached. The lack of a candidate with national appeal signaled that change was coming. Representing the Republicans was Abraham Lincoln, while Stephen Douglas stood for the Northern Democrats. Their counterparts in the South nominated John C. Breckinridge. Looking to find a compromise, former Whigs in the border states created the Constitutional Union Party and nominated John C. Bell.
Balloting unfolded along precise sectional lines as Lincoln won the North, Breckinridge won the South, and Bell won the border states. Douglas claimed Missouri and part of New Jersey. The North, with its growing population and increased electoral power had accomplished what the South had always feared: complete control of the government by the free states.
Secession Begins
In response to Lincoln's victory, South Carolina opened a convention to discuss seceding from the Union. On December 24, 1860, it adopted a declaration of secession and left the Union. Through the "Secession Winter" of 1861, it was followed by Mississippi, Florida, Alabama, Georgia, Louisiana, and Texas. As states departed, local forces took control of federal forts and installations without any resistance from the Buchanan Administration. The most egregious act took place in Texas, where Gen. David E. Twiggs surrendered one-quarter of the entire standing US Army without a shot fired. When Lincoln finally entered office on March 4, 1861, he inherited a collapsing nation.
Election of 1860
Candidate Party Electoral Vote Popular Vote
Abraham Lincoln Republican 180 1,866,452
Stephen Douglas Northern Democrat 12 1,375,157
John C. Breckinridge Southern Democrat 72 847,953
John Bell Constitutional Union 39 590,631
Source: http://militaryhistory.about.com/od/civilwar/a/CivilWarCauses.htm