Germany, while maintaining sufficient forces on the Russian front, was still able to put in the field for its great offensive against France a more numerous body of troops than would have been believed in France. This permitted them to maintain in Alsace, in Lorraine, and in Belgian Luxembourg armies as numerous as those which faced them on the French side, and at the same time to mass the major part of their troops on the right so as to pour into the valley of the Oise their chief invading forces.
This explains why the French left, which was exposed to the offensive of the German right, was obliged to make a rapid retreat, permitting the German armies of General von Kluck and General von Bülow to advance with all speed in the direction of Paris.
The French military staff, as soon as they perceived the danger that threatened, proceeded to a new alignment of forces. As long as this alignment of forces could not be effected the retreat had to continue. As soon as it was accomplished, as soon as General Joffre had his armies well in hand and the situation of his troops well disposed, he checked the retreat, gave the signal for the offensive, and so followed the great Battle of the Marne.
The German plan consisted, therefore, in delivering the main blow through the medium of the right wing of the German forces, consisting of the army of Von Kluck, the army of Von Bülow, and the army of Von Hausen, which were to march with all speed in the direction of Paris.
What plan had the French staff in mind to oppose to this plan of the Germans? Its plan aimed at checking and holding the greatest possible number of Germans by a vigorous offensive in Alsace and Lorraine so as to prevent them from joining the three first German armies which threatened Paris. In support of this offensive of the armies of Alsace and Lorraine, the central French armies attacked in the direction of the Ardennes and Belgian Luxembourg with the object of checking the center of the German armies and then turning toward the west so as to cooperate in the offensive of the French forces which, aided by the British army and the Belgian army, were fighting in Belgium.
The French armies, which are numbered from the right to the left—that is, from the east to the west—comprised: A detachment of the Army of Alsace that was dissolved toward the end of the month of August; the First Army (General Dubail); the Second Army (General de Castelnau); the Third Army (General Ruffey, replaced at the end of August, 1914, by General Sarrail); the Fourth Army (General de Langle de Cary); the Fifth Army (General Lanrezac, replaced in the last days of August, 1914, by General Franche d'Espérey). At the right of this army was stationed the British army under the command of General French.
To what resolution did General Joffre, come? On that memorable evening of the 24th, and on that morning of the 25th, two alternatives presented themselves before him. Should they, rather than permit the enemy to invade the soil of France, make a supreme effort to check the Germans on the frontier?
This first apparent solution had the evident advantage of abandoning to the enemy no part of the national soil, but it had some serious inconveniences. The attack of the German armies operating on the right (Generals von Kluck, von Bülow, von Hausen) were extremely menacing. In order to parry this attack it was necessary considerably to reenforce the French left, and for that purpose to transfer from the right to the left a certain number of army corps. That is what the military call, in the language of chess players, "to castle" the army corps. But this movement could not be accomplished in a few hours. It required, even with all the perfection of organization shown by the French railways during this war, a certain number of days. As long as this operation from the right to the left had not been accomplished, as long as the left wing of the French army and even the center remained without the reenforcement of elements taken from the right, it would have been extremely imprudent, not to say rash, for the French high command to attempt a decisive battle. If General Joffre had risked a battle immediately he would have been playing the game without all his trumps in hand and would have been in danger of a defeat, and even of a decided disaster, from which it might have been impossible to recover.
The second alternative consisted in drawing back and in profiting from a retreat by putting everything in shipshape order to bring about a new grouping of forces. They would allow the Germans to advance, and when the occasion showed itself favorable the French armies, along with the British army, would take the offensive and wage a decisive battle.
It was to this second decision that General Joffre came. As soon as on August 25, 1914, he had made up his mind as to what the French retreat was going to lead he gave orders for a new marshaling of forces and for preparations with a view to the offensive.