At what point did the civil war become inevitable? can you identify any points where a different decision or course of action might have changed the historical trajectory toward war?

The breaking point was in 1828 when the Tariff of Abominations was introduced by the huge support of the Northern representatives of the US Congress. According to the protective plan an enormous tariff was placed on the imported goods from the abroad. That impacted the South's economy and South Carolina stated that they would not enforce the tariffs within their borders. At that point the Civil War became inevitable.

sitting in that one chair where he could get shot at was his worst decision.