Every campaign is conditioned by its political object. The political object in this case was to march upon and to occupy Paris. The political object of a campaign once determined, the size and the organisation of the enemy are calculated and a certain force is brought against it. No much larger force is brought than is necessary: to act in such a fashion would be in military art what paying two or three times the price of an article would be in commerce. The forces of the Allies upon the Rhine and in the Netherlands were, in the opinion of every authority of the time, amply sufficient for their purpose; and more than sufficient: so much more than sufficient that the attitude of that military opinion which had to meet the attack—to wit, the professional military opinion of the French republican soldiers, was that the situation was desperate, nor indeed was it attempted to be met save by a violent and, as it were, irrational enthusiasm.

The second point, the so-called “delay” involved in the sieges undertaken by the Allies, proves, when it is put forward, an insufficient acquaintance with contemporary conditions. Any fortress with a considerable garrison left behind untaken would have meant the destruction of the Austrian or Prussian communications, and their destruction at a moment when the Austrian and Prussian forces were actually advancing over a desperately hostile country. Moreover, when acting against forces wholly inferior in discipline and organisation, an untaken fortress is a refuge which one must take peculiar pains to destroy. To throw himself into such a refuge will always stand before the commander of those inferior forces as a last resource. It is a refuge which he will certainly avail himself of ultimately, if it is permitted to him. And when he has so availed himself of it, it means the indefinite survival of an armed organisation in the rear of the advancing invaders. We must conclude, if we are to understand this critical campaign which changed the history of the world, that Coburg did perfectly right in laying siege to one fortress after another before he began what every one expected to be the necessarily successful advance on Paris. The French despair, as one town after another surrendered, is an amply sufficient proof of the excellence of his judgment.

We approach the military problem of 1793, therefore, with the following two fields clear before us:—

1. In the north-east an advance on Paris, the way to which is blocked by a quadrilateral of fortresses: Mons, Maubeuge, Condé, and Valenciennes, with the subsidiary stronghold of Lequesnoy in the neighbourhood of the last. Mons has been in Austrian hands since Dumouriez’ retreat; Condé is just cut off from Valenciennes by Coburg’s advance, but has not fallen; Valenciennes and the neighbouring Lequesnoy are still intact, and so is Maubeuge. All must be reduced before the advance on Paris can begin. Behind these fortresses is a French army incapable as yet of attacking Coburg’s command with any hope of success. Such is the position in the last fortnight of April.

2. Meanwhile, on the Rhine the French garrison in Mayence is besieged; Custine, the French commander in that quarter, has fallen back on the French town of Landau, and is drawing up what are known in history as the Lines of Weissembourg. The accompanying sketch map explains their importance. Reposing upon the two obstacles of the river on the right and the mountains on the left, they fulfilled precisely the same functions as a fortress; and those functions we have just described. Until these lines were carried, the whole of Alsace may be regarded as a fortress defended by the mountains and the river on two sides, and by the Lines of Weissembourg on the third.

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.

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

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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.