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. [Illustration: 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 Coburg on the north-east. Such in general was the situation upon the eastern frontier at the end of that month of April, 1793. FOUR Let us first follow the development of the northern position. It will be remembered that all Europe was at war against the French. The Austrians had for allies Dutch troops which joined them at this moment, and certain English and Hanoverian troops under the Duke of York who also joined them. At this moment, when Coburg found himself in increasing strength, a tentative French attack upon him was delivered and failed. Dampierre, who was in command of all this French “Army of the North,” was killed, and Custine was sent to replace him. The Army of the North did not, as perhaps it should have done, concentrate into one body to meet Coburg’s threatened advance; it was perpetually attempting diversions which were useless because its strength was insufficient. Now it feinted upon the right towards Namur, now along the sea coast on the left; and these diversions failed in their object. Before the end of the month, Coburg, to give himself elbow room, as it were, for the sieges which he was preparing, compelled the main French force to retreat to a position well behind Valenciennes. It was immediately after this success of Coburg’s that Custine arrived to take command on the Belgian frontier, his place on the Rhine being taken by Houchard. Custine was a very able commander, but a most unlucky one. His plan was the right one: to concentrate all the French forces (abandoning the Rhine) and so form an army sufficient to cope with Coburg’s. The Government would not meet him in this, and he devoted himself immediately to the reorganisation of the Army of the North alone. The month of June and half of July was taken up in that task.