What you should keep in mind is that the Civil War did not emerge out of a vacuum but instead represented the culmination of a longer history of internal turmoil and escalating tensions, stretching back across the 1850s. That internal turmoil was created by the question of slavery's status in the territories (because both the abolitionists and their opponents understood that this question's answer would become critical in determining slavery's future in the United States).
The crisis of the 1850s was already foreshadowed with the earlier Missouri Compromise of 1820. Even at that earlier point in American history, both Northern and Southern states recognized that there was a precarious balance between slave states and free states in the United States (and each side of that division were wary of seeing that balance shift to the advantage of the opposing side). In the decades after the Missouri Crisis, however, abolitionism would grow in the North. Meanwhile, during the 1840s, the United States would dramatically expand all the way to the Pacific Ocean.
This set the stage for the decade of crisis that would follow. Remember, it was from those the territories that new states were created. Thus, the status of slavery in the territories was understood as a question of critical importance, as it would have dictated whether those future states would have joined the Union as slave states or free. Ultimately, this question was understood as one that would determine the very future of slavery within the country. Thus, neither side in this debate could be satisfied with anything less than a complete victory.
What resulted was a history of division and turmoil which would carry across the 1850s and into the early 1860s, escalating over time. The final straw came in the 1860 election, won by Abraham Lincoln (a known opponent of slavery who opposed its continued expansion). The Southern response to this moment was secession, which led to the Civil War.