The American Civil War had both long term and short term causes.
In the long term, the war was caused by major differences between the North and South. The biggest difference was, of course, slavery. But the two sections were also very different in terms of their economic systems and their cultures. These differences caused continual friction between the two regions.
In the short term, we can look at two relatively immediate triggers for the war. The first was the election of Abraham Lincoln. Southerners thought he would be an enemy to them and secession followed soon after his election. The second was Lincoln’s decision to resupply Fort Sumter and the South’s decision to attack the fort.