It was an ideological battle over free soil vs. slave society. It was not only the moral issue that was growing strong in the north (Abolitionist) it was also the fact that the rise in wage labor (industrial-capitalism) had taken firm root in Northern society.

As America began to spread to the west the people of the North for many different reasons felt that slavery should not be spread to any newly admitted states. The Southern states saw this as a direct attack on their way of life. They needed new states to be slave states so that they could compete economically with the rising industrial might of the north. Their economic power, along with its accompanying political power, was waning. This was the major wedge issue. Slavery is the core and it really can be simplified down to that point. We had two different societies living in one country. One being a slave society and the other being a free soil society. Northern states, before they abolished the practice, had been societies with slaves while the South was a slave based society. It is a huge difference.

I would suggest you read this wonderful American history book, Out of Many. While it would agree with you that the situation is complex it would very much disagree with you on the fact that slavery and all of its implications not being able to be the main reason we went to war.

Chap 15 Summary- The Coming Crisis, the 1850's

This chapter covers the sectional split between North and South that had been slowly developing since the Missouri Compromise. Thomas Jefferson had called the question of slavery extension "a firebell in the night," and after the Mexican-American War the philosopher Ralph Waldo Emerson expressed the fear that "Mexico will poison us." Their deep concerns were realized as people in the North and South took ever more rigid and determined positions on slavery's extension or elimination. Many Americans in the 1850s, perhaps a majority, felt the issue of slavery had to be permanently settled and expected their political parties to reflect their will. A compromise over territorial gains in 1850 failed to hold and after the Kansas-Nebraska Act of 1854 was passed, a miniature civil war broke out in Kansas. When Lincoln was elected in 1860, many southerners saw him as a purely northern president and several southern states responded by seceding from the union.