Germany declared war on France on August 3rd, 1914. By September 5th they had marched through Belgium and had crossed into France as far as an east – west line drawn from Paris through Verdun to the German border.
In the process of moving large numbers of troops, both the German and French armies had problems of coordinating movements and operations. Gaps in lines could not be closed and, whether advancing or retreating, had difficulties maintaining contact with their flanking formations. They also had to be concerned about having exposed flanks at the end of lines to avoid being encircled.
The French had moved large numbers of men from the right flank to create the 6th and 9th armies nearer to Paris while maintaining a hold along the Lorraine border at with Germany. They also had to coordinate movements between the retreating Fourth and Fifth Armies, and the British Expeditionary Force (B.E.F.).
The Germans had to sacrifice men during their advance (the 1st & 3rd German armies lost one corps each and the 1st army had given up 3 corps) when they made a distracting feint toward Antwerp, secure central Belgium after they had passed, and to besiege the fortified towns of Givet and Maubeuge. They could not bring in reinforcements quickly from Alsace-Lorraine to their right flank (to the west) because of severe damage to the railroad systems. There was scarce effective control from the German high command. Communication among the three German armies led by Kluck (the 1st), Bulow (the 2nd) and Hausen (the 3rd) was reduced due to their lack of compatibility. To move against Paris itself at this point would be rash as the city was protected by the 6th French Army and the Paris Defence Forces, and would have thinned the German lines too much. Thus, Kluck followed the retreat of the French 5th Army to the south and east of Paris. This would bring the 1st and 2nd German Armies closer.
The French and B.E.F. continued an orderly retreat to the outskirts of Paris. The Germans didn’t anticipate a counterattack from the direction of Paris and so pursued the B.E.F. as far as the Marne River on September 3, 1914.
Allied (red) and German (blue) positions August 23 – September 5, 1914
General Joseph Joffre, Commander of the Allied troops in France, had moved troops from the east to the centre and west in the midst of battle and retreat. He also deployed reserve troops and troops from Lorraine. In so doing he created a new Sixth Army. It linked with the left flank of the BEF, west of the Marne River from Meaux to Pontoise north of Paris.
As the two German armies under Kluck and Bulow moved southeast of Paris to encircle the exposed left flank of the retreating French, they exposed their own right flank to the Allies. On September 4th Joffre ordered the retreat to halt. He then ordered the French 6th Army of 150,000 men and the BEF with 70,000 men to attack the German right flank on the morning of September 6th.