::::Freedom house finds no nation with universal suffrage before 1900, so U.S. may have been the most democratic at the time but certainly less democratic than today. Now Rummel draws an admittedly rather arbitrary lines between democracies and non-democracies. The question if whether the Confederacy fulfills those criteria or not. If the U.S. didn't, then there is no contest, the Civil War was not a war between democracies. That the states voluntarily choose to enter the Confederacy is uninteresting, by doing so they gave their exclusive right to exercise supreme authority in their geographic region, which in politics is considered an absolute requirement to qualify as state. You will not find any example of states belonging to a similar confederacy being accepted as independent states in any international organization. There certainly was no foreign recognition that any of the states were independent nations at the time.