I understand that tension between the Southern and Northern States has been rising for decades, with the issues of slavery, tariffs and so on dividing the nation. My question is-is it possible to say(more or less) when was the tipping point after which tension began to rise exponentially, which act or Supreme Court ruling enraged people enough that peaceful solution was no longer possible?
When seccessionist soldiers fired on Fort Sumter. Up until that point, the federal government - both the Lincoln and Buchannan administrations - had been working on some kind of compromise that would ensure the seceded states' peaceful cooperation. Plans included anything from passing an unrepealable(!) amendment protecting slavery in the states to declaring war on Spain and stealing Cuba to carve out more slave states, protecting the Senatorial balance of power. Secessionist soldiers laying siege to and forcing the surrender of a federal fort could not be passed of firing a warning shot (as rebels had done to the federally-contracted supply shit Star of the West) or as unorganized mob violence (numerous instances in some of the seceded states). This was a sustained, deliberate, dangerous defiance of the federal government and the democratic political process that created it.
Even then, the Civil War as we know it was not inevitable. Virginia seccessionists circumvented the democratic process and declared secession without much public debate and no open voting on the matter. In Missouri, federal military officers seized control of the state before its pro-secession governor could organize secessionist forces. Both states could have easily "switched" loyalties from the paths they historically followed. Contingency is always a factor in history, but the beginning of the Civil War seems to offer an unusual number of plausible alternate scenarios.
Sources: James McPherson, The Battle Cry of Freedom
William W. Freehling, The South vs. The South: How Anti-Confederate Southerners Shaped the Course of the Civil War.