The river Marne is a tributary of the Seine, leaving it at a point east of Paris. It was also famed for being the site of the furthest advance of the imperial German army into France during the Great War. Readers will remember that the Schlieffen Plan (q.v.) was designed to knock France out of the game in six weeks, before the mobilization of the enormous Russian army. Germany would advance in strength through Belgium, bypassing French defences along the German border, and then sweep down to surround Paris before attacking French forces in the rear.
The Schlieffen Plan might have worked had von Moltke (chief of staff) not enfeebled it by transferring forces from the German right-wing to East Prussia, which the Russians had already invaded. Still the Germans made swift progress through Belgium and northern France, leaving the French to make useless and expensive attacks on German forces in Lorraine and the Ardennes.
Von Kluck was in command of the German First Army, and by September I his right-wing crossed the Oise and was thus only 30 miles from Paris. Naturally the French expected an immediate attack on their capital and were amazed when intelligence informed them that von Kluck was instead moving south-east across the Marne to support the Second Army. This exposed his vulnerable flank, which the French attacked on 5 September. The battle of the Marne had begun.
Kluck pulled back to of his army corps from south of the Marne river to meet the attack, leaving a gap of twenty miles between his forces and the German Second Army. Marshal Joffre,
elderly but no fool, seized the day and ordered the British Expeditionary Force through the gap, coming up behind the German First Army. This caused a German retreat as far as the River Aisne, and the great battle of the Marne was over but not before tremendous losses were reported on both sides; allied casualties were estimated at 250,000 and German losses were even bigger.
In the three weeks since the War began each side had lost more than half a million men, mostly officers, and most of them young. But Joffre had won one of the most significant battles since Waterloo, Paris was intact and the Schlieffen Plan defunct. Now there was no hope of Germany knocking France off the board quickly, and the former would have to fight a war on two fronts simultaneously – which she rightly feared. By the end of the year the Western Front extended from the English Channel to Switzerland, the longest in history. Both sides prepared for the coming clash by digging trenches.