Moltke's Military Correspondence, 1870-71 - Published by the Prussian General Staff Department for Military History; First Section, the War to the Battle of Sedan #267533

Black Forest can't be directly defended either by South German States or by Prussia (or Austria). South German Army must fall back either on Mainz (to Prussians) or on Ulm (to Austrians), if any. Retreat to Ulm leads to South Germany being theatre of war or to dealings with France. But retreat to Prussian Army at Mainz makes it impossible for French to advance in South Germany.