Early in December [1860], upon the convening of the United States Congress. “Because it was their platform and projected policies that had provoked the wrath of Southerners, the focus of attention was generally upon Republican lawmakers. President Buchanan sent a message asking the question:

Why is it then that discontent now so extensively prevails?” And the true answer is given
that ‘the long continued and intemperate interference of the Northern people with the question of slavery in the Southern States has at last produced its natural effects. The different sections of the Union are now arrayed against each other and the time arrived so much dreaded by the ‘Father of his Country’ when hostile geographical parties have been formed.

https://books.google.com/books?id=f3tNAAAAYAAJ&pg=PA3&lpg=PA3&dq=Why+is+it+then+that+discontent+now+so+extensively+prevails?&source=bl&ots=4M6K9pJ2mE&sig=L8RpYHsEGAP7DmoirlTOmRmzeU4&hl=en&sa=X&ved=0ahUKEwjCq6HggaTRAhVE8WMKHS3YAYEQ6AEIHTAA#v=onepage&q=Why%20is%20it%20then%20that%20discontent%20now%20so%20extensively%20prevails%3F&f=false

……

What was the war’s basic cause?

The cause of the Civil War must be divided into three parts: First, the issue of slavery and its expansion—which built up over decades and accelerated in the period between 1846 and 1860—came to a head in the presidential election of 1860, causing the deep South states to secede when Abraham Lincoln’s election convinced them they had lost control of the national government and, therefore, of slavery’s fate within the Union. Second, Lincoln’s determination not to compromise on the issue of slavery’s expansion. Third, Lincoln’s dedication to resupply rather than abandon Fort Sumter, and the decision of Jefferson Davis’ administration to fire on federal troops at the South Carolina fort. The final catalyst, as opposed to the long-term cause, was the crisis over Fort Sumter.
Civil War historian James M. McPherson
http://www.historynet.com/interview-with-civil-war-historian-james-mcpherson.htm

……….

The burning issue that led to the disruption of the union was the debate over the future of slavery. That dispute led to secession, and secession brought about a war in which the Northern and Western states and territories fought to preserve the Union, and the South fought to establish Southern independence as a new confederation of states under its own constitution.
Causes Of The Civil War
The Events That Caused The American Civil War
www.historynet.com/causes-of-the-civil-war