This may be illustrated by reference to particular campaigns.  In 1792, when the Duke of Brunswick invaded France, she had no armies competent to her defence.  Their numbers upon paper were somewhat formidable, it is true, but the license of the Revolution had so loosened the bonds of discipline as to effect an almost complete disorganization.  “It seemed, at this period,” says the historian, “as if the operations of the French generals were dependent upon the absence of their enemies:  the moment they appeared, the operations were precipitately abandoned.”  But France had on her eastern frontier a triple line of good fortresses, although her miserable soldiery were incapable of properly defending them.  The several works of the first and second lines fell, one after another, before the slow operations of a Prussian siege, and the Duke of Brunswick was already advancing upon the third, when Dumourier, with only twenty-five thousand men, threw himself into this line, and by a well-conducted war of positions, placing his raw and unsteady forces behind unassailable intrenchments, succeeded in repelling a disciplined army nearly four times as numerous as his own.  Had no other obstacle than the French troops been interposed between Paris and the Prussians, all agree that France must have fallen.

In the campaign, of 1793, the French army in Flanders were beaten in almost every engagement, and their forces reduced to less than one half the number of the allies.  The French general turned traitor to his country, and the National Guards deserted their colors and returned to France.  The only hope of the Republicans, at this crisis, was Vauban’s line of Flemish fortresses.  These alone saved France.  The strongholds of Lille, Conde, Valenciennes, Quesnoy, Landrecies, &c., held the Austrians in check till the French could raise new forces and reorganize their army.  “The important breathing-time which the sieges of these fortresses,” says an English historian, “afforded to the French, and the immense advantage which they derived from the new levies which they received, and fresh organization which they acquired during that important period, is a signal proof of the vital importance of fortresses in contributing to national defence.  Napoleon has not hesitated to ascribe to the three months thus gained the salvation of France.  It is to be constantly recollected that the Republican armies were then totally unable to keep the field; that behind the frontier fortresses there was neither a defensive position, nor a corps to reinforce them; and that if driven from their vicinity, the capital was taken and the war concluded.”

In the following year, 1794, when France had completed her vast armaments, and, in her turn, had become the invading power, the enemy had no fortified towns to check the progress of the Republican armies; which, based on strong works of defence, in a few weeks overran Flanders, and drove the allies beyond the Rhine.