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.
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| Strategic situation in early summer of 1793. Mayence besieged, Condé and Valenciennes about to be besieged. Conditions of the double advance on Paris. |
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
