By early September, the Franco–British forces outnumbered the Germans who were exhausted after a month-long campaign, had outrun their supply lines, and were suffering shortages.

On 3 September the military governor of Paris, Joseph Simon Gallieni, perceived that the German right flank was vulnerable and positioned his forces to attack.

Both sides next commenced reciprocal operations to envelop the northern flank of their opponent in what became known as the Race to the Sea which culminated in the First Battle of Ypres and led to a bloody four-year long stalemate of trench warfare on the Western Front of World War I.

[10][11] By contrast, the French commander, Joseph Joffre, was a whirlwind of activity (although insisting on fine dining and an uninterrupted eight hours of sleep every night).

The French did not fortify their north western border with Belgium as they did not expect the Germans to attack there and also feared being accused of violating Belgian neutrality—and thereby losing British participation in the war.

Germany, however, had no compunctions about violating Belgian neutrality as its objective was to win the war quickly before Great Britain could intervene decisively.

To historian Herwig, the Schlieffen Plan, the violation of Belgian neutrality, and the German invasion of France in 1914 were "an all-or-nothing throw of the dice, a high-risk operation born of hubris and bordering on recklessness.

He abandoned the aggressive Plan XVII and instead proclaimed that the French armies were "forced to take defensive action...to wear down the enemy's strength and resume the offense in due course."

[25] Demands for more soldiers on other fronts in the war (and possible over-confidence) resulted in Moltke reducing the number of German attackers in France by 200,000 or more men in August.

[29] Moltke realized he did not have sufficient forces to carry out the Schlieffen Plan which envisioned the 1st army of Kluck encircling Paris to the west and south.

Instead, interpreting the order broadly (or disobeying it), Kluck turned his line of march from south to southeast, becoming closer but not in echelon with Bülow, and on 3 September his forces crossed the Marne River 60 km (37 miles) east of Paris, the first Germans to do so.

[32] He was unaware that a new French army, the 6th of Joseph Gallieni and Maunoury, was guarding Paris to his west and he discounted the British Expeditionary Force which had apparently abandoned the battlefield.

Then, on 5 September, Joffre journeyed to BEF headquarters for discussions which ended with him banging his hand dramatically on a table while shouting "Monsieur le Maréchal, the honour of England is at stake!"

This involved a withdrawal of Kluck's forces who had crossed the Marne River to the south and now had to march 130 km (81 miles) in two days to reach positions facing the French.

Gallieni commandeered about six hundred taxicabs at Les Invalides in central Paris to carry soldiers to the front at Nanteuil-le-Haudouin, fifty kilometres away.

[42][43][44] The arrival of six thousand soldiers by rail, truck, and taxi has been described as critical in preventing a possible German breakthrough against the 6th Army.

"[50][51][52] Bülow's 2nd army south of the Marne on 6 September was as worn and depleted as Kluck’s, having marched 440 km (270 miles) since leaving Germany and having suffered more than 26,000 casualties and soldiers felled by illnesses.

On 7 September, Bülow ordered his right wing to retreat 15 km (9.3 miles) to the Petit Morin River after attacks by the French 5th army of Franchet d'Esperey, called "Desperate Frankie" as a compliment by the British.

French air reconnaissance observed German forces moving north to face the Sixth Army and discovered the gap.

The BEF advanced on 6–8 September, crossed the Petit Morin, captured bridges over the Marne, and established a bridgehead 8 kilometres (5 mi) deep.

The 1st Army was poised to assault the city of Paris and, hopefully, win the war, but Kuhl acceded to Hentsch and informed Kluck.

The Germans were pursued by the French and British, although the pace of the exhausted Allied forces was slow and averaged only 19 km (12 mi) per day.

[71]On 14 September, German military authorities informed Kaiser Wilhelm II that "Moltke's nerves are at an end and [he] is no longer able to conduct operations."

Following the battle and the failures by both sides to turn the opponent's northern flank during the Race to the Sea, the war of movement ended with the Germans and the allied powers facing each other across a stationary front line of trenches and defenses that remained nearly stable for four years.

[77] It was his orders that prevented Castelnau from abandoning Nancy on 6 September or reinforcing that army when the pivotal battle was unfolding on the other side of the battlefield.

[84] Tuchman wrote that Kluck explained the German failure at the Marne as …the reason that transcends all others was the extraordinary and peculiar aptitude of the French soldier to recover quickly.

But that men who have retreated for ten days, sleeping on the ground and half dead with fatigue, should be able to take up their rifles and attack when the bugle sounds, is a thing upon which we never counted.

[85]Richard Brooks in 2000 wrote that the significance of the battle centres on the fact that the failure of the Schlieffen Plan forced Germany to fight a two-front war against France and Russia—the scenario its strategists had long feared.

[92] Sumner cites the same overall casualty figure for the French for September as Herwig from Armées Françaises, which includes the losses at the battle of the Aisne, as 213,445 but provides a further breakdown: 18,073 killed, 111,963 wounded and 83,409 missing.

The 2nd and 9th Cavalry divisions were dispatched as reinforcements the next day but before the retirement began, the French attack reached Carlepont and Noyon, before being contained on 18 September.