The Civil War had many different causes, but the single largest contributor was the debate over slavery. Slavery had been an issue even before the colonies united to fight Britain during the revolutionary war, and the years of debate over the issue added to the tensions between north and south that finally exploded into the Civil War.
The abolition movement compounded the slavery issue by adding a moral imperative. Their demonization of the southern slave owner along with their hero worship of “crazy” individuals like John Brown, who tried to insight an armed slave revolt, made the south more and more defensive of the institution and less and less likely to compromise.
Another contributing factor was regional differences. The north, with its industrialized economy, was constantly asking for protective tariffs which the south, with its cash crop driven agrarian economy, felt were disruptive to trade. They also constantly battled over funding for roads, canals and railroads which the south saw as superfluous but the north desperately wanted.
Money was also a factor. Much of the financial wealth was concentrated in northern banks, which southerners were hesitant to invest in due to their location. Southerners were much more likely to trust state run financial institutions because of their natural distrust of “yankee financiers”
If you had to name a single moment that caused session, the straw that broke the camel’s back would have to be the election of Lincoln in 1860. Lincoln ran on a platform of stopping the spread of slavery, which was totally unacceptable for southerners since they depended on equal representation in the Senate. Without it, they would not be able to block any anti-slavery bills that came to the floor of congress, and the eventually death of their way of life would be assured. When Lincoln was elected, they saw the writing on the wall, and began to secede. The first shots were fired at Fort Sumter in 1861.
Reconstruction refers to the period directly after the Civil War. This was a period of rebuilding the south both physically and socially. Physically, the war devastated the south. Their major cities were in ruins. Their trade networks were totaled. Their population was dead, wounded or terrorized. A serious rebuilding of southern infrastructure needed to begin along with the integration of now free African Americans. A new social order needed to be created where African Americans were now free and equal to their former white masters. This was a particularly violent period in the south, and fights, riots and racially-motivated terrorism was common as whites tried to assert their dominance over their former property.