In hindsight, we can now say that it was the Dredd Scott decision by the Supreme Court in 1857 that made the war inevitable. That ruling essentially erased any distinction between slave and free states, decreed slavery could be anywhere. That was going to be impossible for the North to tolerate.

However, the question as to when people of the time knew war was going to happen is much harder to guess. There had been many decades of compromises and deals worked out within Congress to avoid a confrontation. Throughout the 1850's, the South had gotten accustomed to having its way most of the time. Its representatives in Congress were used to intimidating and threatening their Abolitionist opponents, even challenging them to duels, and many of them assumed that the North would bend to their demands even as they began to secede. And there were plenty of voices in the North, including President James Buchanan, who had no interest in starting a war, would have been happy to come up with some sort of compromise that gave the South much of what it wanted. It would not be until the Southern forces began to fire on Ft Sumter that a conflict seemed inevitable. But even then, the South's strategy during the War was not to defeat the North but to fight it enough to get it to settle for terms agreeable to the South. Lincoln would therefore have plenty of Democrats calling for a negotiated settlement throughout the war, and Southerners calling for a peaceful end to the conflict- if they could keep their sovereignty and their slaves. It would have been possible for many US citizens to assume that, even after it started, the war would quickly stop.