Don't miss the big stories. Like us on Facebook. "War is God's way of teaching Americans geography," wrote Ambrose Bierce, whose bitter insights were shaped in large part by a terrible war. As a soldier in the Civil War, Bierce witnessed scenes of slaughter over what was, in important ways, a fight to control rivers. You can read it in the names of the great Union armies: the Army of the Ohio, the Army of the Potomac, the Army of the Tennessee, the Army of the Cumberland. The battle over slavery was also a battle to preserve free commerce from east to west on the Ohio River and from north to south on the Mississippi.