So, speaking as a geologist... this would not necessarily destroy human civilization, because most countries still grew their own food and because the US (which would be most heavily affected) was not yet the breadbasket of the world. It would cause significant famines, though, and it'd be a particularly impressive country which got through the experience without undergoing a major transformation.
The world's geological experts at this time know of volcanoes and have vague ideas on how the eruptions take place, but something this big will be outside their understanding and experience - comparisons might be drawn to the 1783 Laki eruptions in iceland, but this would be on a far larger scale.
The Civil War would effectively be cancelled. Quite apart from dangerous to deadly ash falls reaching as far east as Kentucky (this is the area where the ash itself would cause serious risk), both North and South would have far bigger problems on their hands. This probably means an independent Confederacy, but they'd almost immediately have cause to seriously regret it.