Read the history books closer, my FRiend. That is EXACTLY what happened last time around. The last war was, literally, "brother against brother," and so will be the next one.
The War of Independence (Revolutionary War) also was brother against brother with the population split into thirds: one third indifferent, one third against (Tory) and one third FOR revolt. Many of the Tories emigrated to Canada during and after the war.