First, we should note that the Civil War became more inevitable as time went on. In other words, it is more likely that the war could have been prevented in 1855 than in 1861 and more likely still that it could have been prevented in 1844. This is because the tensions between the North and South built up somewhat gradually over the years, creating more and more pressure towards a war.
If you are to argue that the war was inevitable, you can say that the North and South were simply too different to be able to remain together in one country. You can use Abraham Lincoln’s line that a “house divided against itself cannot stand.” The North and the South had different economic systems (including the fact that the South had slaves). This meant they had different economic and political needs. The North and the South had different social systems. These differences made the two sides see one another as alien and therefore dangerous.
Because the two sides did not think they had much in common and yet were part of the same country, you can argue, a civil war was inevitable.