The election of Abraham Lincoln in 1860 was the final straw in a decades-old buildup of ever-widening tension between the Southern states and the North over the institution of slavery. Though Lincoln was hardly the most radical candidate that the abolition-leaning Republican party could have nominated, not one Southern state voted for him. Those with power in the South loathed him as the man they feared would end slavery.
The United States, still a new nation and a great experiment, had tried desperately to stay united despite the growing and severe polarization over slavery. The more harshly the North condemned slavery as immoral and called for its immediate end, the more firmly the Southerners dug in. By the time 1860 rolled around, the South had moved from understanding slavery as a "necessary evil" to promoting it as a positive good that was better for the Black slaves than freedom.
The two ideological poles, made impossible to reconcile largely due to the extremism and delusion thinking of the South, had grown so large by 1860 that there simply was no way to bridge the gap. The country could no longer keep up the pretense that it could both simultaneously support and condemn slavery and somehow make this situation work.
Lincoln's election signaled the end to attempts at compromise. His win was more or less a foregone conclusion, as the Democratic Party had split and run more than one candidate, almost guaranteeing Lincoln the presidency. His election acted as a symbolic rallying point around which the Southern plantation owners could justify the split they already had decided was coming.