Answer:
1. election of Abraham Lincoln to the presidency
Step-by-step explanation:
The southern states at that time relied heavily on slavery to sustain their economy. Large portion of their income came from agriculture sectors and they use slave labors for their operation.
Abraham Lincoln was openly known for anti-slavery stance even before he got elected.
As soon as Lincoln won the presidency, the southern states felt heavily threaten because they knew it only a matter of time before he created a policy to abolish slavery. This caused the states to secede from the union, which become one of the main reasons why the civil war happened.