In what sense was the US Civil War inevitable?

The US Civil War was not inevitable. Generally speaking, despite the differences of opinion on a large issue, that of slavery, the election of Abraham, Lincoln was the tinderbox that gained momentum as his day of inauguration came closer and closer. The failure of a united Democratic Party was the prime factor in Abraham Lincoln's election. For all practical purposes, the Democrats had three candidates on the ballot instead of one in the 1860 presidential election.Time and patience among wrong thinking Southerners combined to force the issue. Lincoln, a strong Unionist, guaranteed he had no intention to undermine the Supreme Court and hurt the huge plantation owners. Southerners also had a blind eye towards the history of the USA. Had they been "thinking", it would have given them the realization that Lincoln might easily have been a one term president. In the absence of a war, there was every chance that in 1864, a "friendly" Democrat could have won the 1864 presidential election.Lincoln won a substantial amount of Electoral College Votes, however, if a Democrat such as Stephan A. Douglas was the sole Democratic nominee it's a good opinion that Abraham Lincoln with 31% of the popular vote would not have been elected. The Southern plantation owners had it all in the "bag" so to speak with the US Supreme Court in 1859 legalizing slavery.

With that said, like the British, and with a hand from Adam Smith and his Wealth of Nations, a bit of forward looking vision would have shown that slavery was not a good investment. This lack of understanding in both the South and among abolitionists in the North increased the tension on this crucial issue.

Aside from that, the morality issue would have forced a British style of gradually compensating owners for their slaves. The difficulty, which Lincoln had already realized was that the integration of 4+ million slaves into US society would be a difficult task. That's why he and slave owner, Senator Henry Clay, his friend and political mentor, had some hopes for the American Colonization Society.

Hope, is all they had. With that said, in the worst possible manner African Americans were step by step integrated into society after a war that should never had been needed.

A gradual plan of freedom and compensation would have eased the race relation issues. Having slavery end as a result of the largest mistake in US history, the US Civil War, was the worst result possible.

The 620,000 deaths, a wrecked Southern economy, and all that went with it ended slavery the wrong way and everyone in the USA suffered from it. America overcame that for sure and growth without the South expanded. But imagine the better growth with a united country.

Probably two reasons are more important than others. First, the issue of slavery was very divisive between the north and south even before the Declaration of Independence was issued. Second, the new country became divided philosphocally about the powers between the federal and state governments. This led to sectional interests between the north and south and slavery was only one difference. Southern states believed states had rights to overrule federal laws. Southern states refused to sign the Declaration of Independence unless the mention of abolishing slavery was removed. The issue arose again in the Constitutioanl Convention. The north wanted it abolished but the south did not. A compromise was reached as stated in Article 1, Section 9, Clause 1. Essentially, it put the issue of abolition off till 1808, so it was destined to arise again. After that year northern states put pressure on southern ones to abolish slavery but the southern states refused and resented other states interfering in their business. This clash of principles got more and more intense until the southern states had enough and declared their own independence from the United States government.