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.