What do you think could have been done, if anything, to prevent the civil war? could simple compromise or political debate between the two sides have eventually ended slavery without a war?

Although one could argue that a nonviolent solution to the differences between the South and the rest of the Union was possible, I believe that it would have been very difficult and unlikely, especially as the Union continued to grow and admit new states. As the United States grew, each new state became a political (and at times literal) battleground, especially prior to its full statehood, regarding the issue of slavery. 
This battle for slavery in the new states/territories was exacerbated by Congress pushing, with success, for full abolition of slavery in the entire country. This was simply unacceptable for southern states at the time, whose society, culture, and, most considerably, economy were built on slavery as a vital institution. They were unwilling to accept the imposition of this law, and all that it implied regarding the equal rights of African Americans and how it would change the plantation-based southern economy.
 
The history of how the US came to independence, and the history of the southern states as the loudest opponents of federalism, created a situation in Southern States saw war and secession as much a duty as an optimal choice in the face of a domineering, abolition-pushing federal government. Given this environment in the Antebellum United States, war would have been incredibly difficult to avoid.