The idea that the sides were split geographically has some truth, but is overstated and a result of looking at the decisions of state governments -- there were supporters of the Union cause in states that seceded (and, in fact, there were union regiments raised from every state in the Confederacy except South Carolina), and supporters of the rebel cause in states that remained in the Union. The illusion of a clean geographic split is the result of the fact that a state government's choice to secede and join the confederacy or not was starkly binary.
Dunno why you got downvoted, but that's actually a much more legit objection to the "guns are a last defense against tyranny" argument than "ZOMG DRONEZ". My answer is, I have no idea, and I hope I never find out.
You may be interested in the Korean movie, "Tae Guk Gi," which illustrates exactly what you're talking about and occurred during your parents' or grandparents' lives.
Err... the movie shows what happens when the government forces people to shoot the bad guys. When there are two governments (The Korean War), brothers end up shooting at each other.
To be sure, there were two governments in the US Civil War as well. I'd say it's a pretty rare situation where a unified government turns citizens against each other with guns. Maybe The Great Leap Forward and the like, but even that was maybe more prosecutorial.
You mean like the Revolutionary and Civil wars in the US? Yes, that exact thing does sometimes occur. Historically it occurs frequently all around the world. At any given time, there is a civil war going on somewhere in the world that involves just such conflict.
