 Military planners in Paris understood that the upper Rhine Valley , the south @-@ western German territories , and Danube river basin were strategically important for the defense of the Republic . The Rhine was a formidable barrier to what the French perceived as Austrian aggression , and the state that controlled its crossings controlled the river itself . Finally , ready access across the Rhine and along the Rhine bank between the German states and Switzerland , or through the Black Forest , gave access to the upper Danube river valley . For the French , control of the Upper Danube or any point in between , offered an immense strategic value and would give the French a reliable approach to Vienna . 
