In the last one, the Union and Confederacy were geographically distinct. Although large segments of the population may have disagreed with the side they wound up on, still, most Confederates lived in the South, and most Unionists lived in the North. The West was up for grabs, but it was relatively unpopulated. This distribution made possible the raising of large conventional armies fighting a conventional military conflict, and political subdivision of the continent into relatively contiguous and uniform regions.

The ideological subdivision now threatening the union is not highly correlated geographically. It manifests itself in class and economics, urban vs rural, and there are powerful racial and religious divisions. The factor which it seems most determines how a region will vote, at any areal resolution, is its population density.

by state and county

Every state, city, and neighborhood is a potential battleground. Even in those regions of the country which are recognized to support one side predominantly, such as the South, the split is at most on the order of 60-40. Its hard to see how secession could happen in a scenario like this. Every town and and every street would have its own Civil War. The middle east has its tribes and militias, we have criminal gangs, organized crime, and militias of our own.

A new American Civil War would look more like 2012 Syria or the Balkans in the 1990s than like 1860s America. Even in those districts where one side was firmly in the majority, they would have substantial numbers of the Enemy living in their midst. And when one side actually prevailed, it would seek vengeance and punishment against the other when the conflict finally ended.

The last Civil War did not permanently resolve the conflicts which caused it. It left a country that was bitter and angry, divided, uglier than it had been before. The legacy of another Civil War would be much worse.