Compromise between slave and free states collapsed with secession. Why did compromise fail at this moment?

In the 1850s, many Americans supported popular sovereignty as the best solution to the explosive question of slavery in the western territories. Why was this solution so popular, and why did it ultimately prove inadequate?

What caused the realignment of the two-party system in the 1840s and 1850s? How did this contribute to the coming of the Civil War?

Abraham Lincoln believed that he had staked out a moderate position on the question of slavery. Why, then, did some southern states determine that his election necessitated secession?