Why was the American Civil War inevitable?

I think a more appropriate answer would be that the Civil War was inevitable from the founding of separate colonies in the Chesapeake area and New England in the 1600's. The southern economy was agricultural because the area had good soil and long growing seasons which lent itself to large scale agronomy. This situation did not exist in the North, where thin soil and short growing seasons made shipping and manufacturing the only reasonable method of economic success.

Differences between the two arose not so much over slavery (although it was always the gigantic elephant in the room) as over issues such as the Tariffs imposed to protect Northern industry which were injurious to the southern economy. The first serious threat of secession from the South was the nullification crisis during the Jackson administration. It should be noted, however, that those who opposed the tariff feared that the federal government would one day arrogate to itself the power to abolish slavery.

So the two areas were so different from the outset that the Civil War (properly known as the War of the Rebellion) was likely inevitable from the founding of the colonies. The two areas were too different and evolved differently.

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.

Was the American Civil War inevitable or could it have been avoided?

No actions that we humans take are inevitable.  All can be avoided.  There were many ways the Civil War could have been avoided, but it is not surprising that they were not taken and the war happened.

The war could have been avoided most easily if one side or the other had simply given in.  The North or the South could have given in on the issue of slavery in the territories.  This would have prevented the war, but it is not likely that either side would ever have done this.

The war could have been avoided if Lincoln had simply allowed the South to secede.  Instead, he chose to fight to preserve the Union.  Again, though, it is at least somewhat unlikely that any President could have gotten away with simply allowing the South to secede.

In short, the war could conceivably have been avoided in a number of ways.  However, it would have been difficult for the leaders to actually take the actions that would have avoided war.