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 A reader unacquainted with military history may ask why the obstruction was not drawn upon the line of the Prussian advance on Paris. The answer is that the presence of a force behind fortifications anywhere in the neighbourhood of a line of communication is precisely equivalent to an obstacle lying right upon those lines. For no commander can go forward along the line of his advance and leave a large undestroyed force close to one side of that line, and so situated that it can come out when he has passed and cut off his communications; for it is by communications that an army lives, especially when it is marching in hostile country.

Custine, therefore, behind his Lines of Weissembourg, and the besieged garrison in Mayence, correspond to the barrier of fortresses on the north-east and delayed the advance of the Prussians under Wurmser and Brunswick from the Rhine, just as Condé, Valenciennes, and Maubeuge prevented the advance of