I agree with Stephen A. Douglas, that if we had only waited long enough, the West would have grown populous enough to become a factor, and would have outweighed the other two sections of the country (and could have then put enough pressure on the South to eradicate the last of a non-profitable institution, slavery).
But there were probably also powers in Europe that wanted to see the USA broken up (including hints at making the West independent, under Fremont). Bismark said the greatest mistake Europe made was in letting the USA get back together. And France wanted to grab Mexico in the bargain.
So, instead of the USA getting back together peaceably after the war, we might have been divided into THREE sections--all in perpetual war with one another, and with European interventionist forces in Mexico and Canada.
They don't lend themselves to Roman numeral sequencing (Independence War I, followed by IWII? Nope), and the current names have become accepted through use, and I won't get noisily insistent on the subject :)
But I will bring it up when an opportunity such as this one appears...
I agree the conflict between the federal government and the eleven states who tried leave the US was not a civil war. It was a war of secession - of rebellion - by a one part a country attempting to leave a federal union.
The leaders of the states attempting to leave the US made no attempt to resolve their differences with the federal government by political means.
And there is ample evidence that Lincoln - who was elected by a minority of the popular vote - would have been willing to accept almost any compromise that would have kept the country together.
Consequently, the responsibility for the outbreak of war falls on the leaders of the eleven states.