It's really very simple. The south seceded to protect slavery from the threat to it that they saw in Lincoln's election. The north went to war to preserve the union and to avenge the attack on Sumter. But in the end, you can't deny the obvious--that the war ended slavery.
The 13th A. ended slavery. The war ended freedom and independence from a central government. Slavery wasn't an issue for the North. It was one of many for the South. Why would the South wage such a vast and reaching war over an issue that was already settled in the US Constitution? Just for the expansion? Why would a slave owner risk "life, fortune and honor" for an issue that was already settled for him? Why would hundreds of thousands of non-slave owners risk the same thing? For expansion of slavery to the West?