Read Robert Citino's The German Way of War, one of several books he's written on the general subject: it was simply not possible for the German/Prussian military to 'sit behind' any defensive line as a strategy. Ever since the Great Elector and the 1650s they had always attacked, trying to end the war quickly. German military history, in fact, is full of army, corps or division commanders who disobeyed orders and attacked, sometimes disastrously, who were never condemned for their aggressive action, because aggressive action was the Default Mode for a German commander at all levels.
Also, Germans/Prussians had been fighting in the east (Poland and further) since 1807: they knew perfectly well how difficult it was to sustain an offensive on the more primitive road and rail network there. It was only after the attack on France collapsed in 1914 that they started considering an offensive in the east, and then even with substantial superiority in firepower and mobility it took them 3 years to drive Russia to collapse. No German commander or government had ever planed for a multi-year war: it simply wasn't part of their strategic thinking or mind-set.