From Columbia website:

Shenandoah covers the remarkable Valley Campaign of May/June 1862. Led by Thomas 'Stonewall' Jackson, a smaller Confederate army with audacious marching and fighting, paralyzed and defeated three enveloping Union armies. Military students worldwide study Jackson's strategy and tactics to this day.

The CSA player has the advantage of central position, but must use it aggressively to prevent the Union armies from combining against him. The USA player has a clear advantage in numbers, but must cope with a command system that penalizes the wide separation of his three forces.