Civil wars aren't always fought with armies.
They are also waged on city streets and country roads, college campuses, on picket lines and police stations, in army barracks and back alleys, by civilians, and troops in mufti. It can be fought with arson, assassination, sabotage, torture, and just having people conveniently disappear. It will be all about terror. Think of Argentina, Lebanon, Algeria, Ireland, Cuba.
Our Civil War was very nineteenth century; armies, uniforms, flags, cannon and horses, telegraphs and railroads, steamboats and Minie balls. Cavalier officers in plumed hats on dashing chargers. It was fought on a continental scale, one region against another.
Our war will be between neighbors, relatives, colleagues. But mostly, perfect strangers, and mostly anonymous.