If it is the US civil war, indeed slavery was one reason, but tensions between Northern and Southern politicians is another problem as they couldn't effectively compromise over how to deal with slavery and 1850 Compromise, Missouri Compromise etcetra were not effective. Also, Lincoln's election as President is a cause as well as he was suspected to be against slavery as he was a Republican: as the Southern states succeeded in response to Lincoln's election, the North wanted the South to return = war. Also, arguments over who owned Fort Sumter is another cause. Other causes such as the break down of the two party system can be also claimed to have caused the civil war.