There was another necessity of no less importance. The retreat must be covered on the east. After the reverse of Morhange–Sarrebourg, this was a continual source of anxiety. On August 25, the German Armies of Lorraine, now reinforced, were hammering at the circle of hills called the Grand Couronné of Nancy, and were upon the Moselle before the Gap of Charmes. Belfort and Epinal were safe, and Verdun was not yet directly threatened. Very little consideration of the rectangular battle front——the main masses ranged along the north, while a line of positions naturally and artificially strong favoured the French on the east——would lead to the further conclusion: to stand fast along the east, as cover for the retreat from the north. Castelnau and Dubail, therefore, were asked to hold their critical positions at any cost. At the same time, Mulhouse and the northern Vosges passes were abandoned; Belfort, Epinal, and even Verdun were deprived of every superfluous man, in order to meet the main flood of invasion. The evacuation of Verdun and Nancy was envisaged as a possibility. The line Toul–Epinal–Belfort could not be lost without disaster.
Such were the three chief conditions affecting the extent of the strategic retreat. Conditions are, however, to be made, not only suffered; and General Joffre had no sooner got the retreat in hand than he set himself to the constitution of a new mass of manœuvre by means of which, when a favourable conjuncture of circumstances should be obtained, the movement could be reversed. The simultaneous disengagement and parallel withdrawal of four armies, with various minor forces, over a field 120 miles wide and of a like depth, was an operation unprecedented in the history of war. The pains and difficulties of such a retreat, the danger of dislocation and demoralisation, are evident. Its great compensation was to bring the defence nearer to its reserves and bases of supply, while constantly stretching the enemy’s line, and so weakening his striking force. This could not, of course, be pure gain: the French and British Armies lost heavily on the road south by the capture of laggards, sick, wounded, and groups gone astray, as well as in killed and men taken in action. The Germans lost more heavily in several, perhaps in most, of the important engagements, and they were much exhausted when the crucial moment came. On the other hand, the Allies were constantly picking up reinforcements; while the enemy had to leave behind an army of occupation in Belgium, and large numbers of men to reduce Maubeuge, to garrison towns like Lille, Valenciennes, Amiens, St. Quentin, Cambrai, Laon, Rethel, Rheims, to terrorise scores of smaller places, and to provide guards and transport for ever-lengthening lines of communication.