The First Battle of the Marne marked the end of the German sweep into France and the beginning of the trench warfare that was to characterise World War One.
Germany's grand Schlieffen Plan to conquer France entailed a wheeling movement of the northern wing of its armies through central Belgium to enter France near Lille. It would turn west near the English Channel and then south to cut off the French retreat. If the plan succeeded, Germany's armies would simultaneously encircle the French Army from the north and capture Paris.
A French offensive in Lorraine prompted German counter-attacks that threw the French back onto a fortified barrier. Their defence strengthened, they could send troops to reinforce their left flank - a redistribution of strength that would prove vital in the Battle of the Marne. The German northern wing was weakened further by the removal of 11 divisions to fight in Belgium and East Prussia. The German 1st Army, under Kluck, then swung north of Paris, rather than south west, as intended. This required them to pass into the valley of the River Marne across the Paris defences, exposing them to a flank attack and a possible counter-envelopment.
On 3 September, Joffre ordered a halt to the French retreat and three days later his reinforced left flank began a general offensive. Kluck was forced to halt his advance prematurely in order to support his flank: he was still no further up the Marne Valley than Meaux.
On 9 September Bülow learned that the British Expeditionary Force (BEF) was advancing into the gap between his 2nd Army and Kluck. He ordered a retreat, obliging Kluck to do the same. The counterattack of the French 5th and 6th Armies and the BEF developed into the First Battle of the Marne, a general counter-attack by the French Army. By 11 September the Germans were in full retreat.
This remarkable change in fortunes was caused partially by the exhaustion of many of the German forces: some had marched more than 240km (150 miles), fighting frequently. The German advance was also hampered by demolished bridges and railways, constricting their supply lines, and they had underestimated the resilience of the French.
The Germans withdrew northward from the Marne and made a firm defensive stand along the Lower Aisne River. Here the benefits of defence over attack became clear as the Germans repelled successive Allied attacks from the shelter of trenches: the First Battle of the Aisne marked the real beginning of trench warfare on the Western Front.
In saving Paris from capture by pushing the Germans back some 72km (45 miles), the First Battle of the Marne was a great strategic victory, as it enabled the French to continue the war. However, the Germans succeeded in capturing a large part of the industrial north east of France, a serious blow. Furthermore, the rest of 1914 bred the geographic and tactical deadlock that would take another three years and countless lives to break.