Already on September 3, General J.-S. Gallieni, the military governor of Paris, had guessed the significance of the German 1st Army’s swing inward to the Marne east of Paris. On September 4 Joffre, convinced by Gallieni’s arguments, decisively ordered his whole left wing to turn about from their retreat and to begin a general offensive against the Germans’ exposed right flank on September 6. The French 6th Army, under M.-J. Maunoury, forewarned by Gallieni, had actually begun attacking on September 5, and its pressure caused Kluck finally to engage the whole 1st Army in support of his right flank when he was still no farther up the Marne valley than Meaux, with nothing but a cavalry screen stretched across the 30 miles between him and Karl von Bülow’s 2nd Army (at Montmirail). While the French 5th Army was turning to attack Bülow, the BEF (between the 5th and the 6th armies) was still continuing its retreat for another day, but on September 9 Bülow learned that the British too had turned and were advancing into the gap between him and Kluck. He therefore ordered the 2nd Army to retreat, thus obliging Kluck to do likewise with the 1st. The counterattack of the French 5th and 6th armies and the BEF developed into a general counterattack by the entire left and centre of the French army. This counterattack is known as the First Battle of the Marne. By September 11 the German retreat extended to all the German armies.

There were several reasons for this extraordinary turn of events. Chief among them was the utter exhaustion of the German soldiery of the right wing, some of whom had marched more than 150 miles (240 kilometres) under conditions of frequent battle. Their fatigue was ultimately a by-product of the Schlieffen Plan itself, for while the retreating French had been able to move troops by rail to various points within the circle formed by the front, the German troops had found their advance hampered by demolished bridges and destroyed rail lines. Their food and ammunition supply was consequently restricted, and the troops also had to make their advance by foot. Moreover, the Germans had underestimated the resilient spirit of the French troops, who had maintained their courage and morale and their confidence in their commanders. This fact was strikingly evidenced by the comparatively small number of prisoners taken by the Germans in the course of what was undeniably a precipitous French retreat.

Meanwhile, the assault by the German 6th and 7th armies on the defenses of the French eastern frontier had already proved a predictably expensive failure, and the German attempt at a partial envelopment pivoted on Verdun was abandoned. The German right wing withdrew northward from the Marne and made a firm stand along the Lower Aisne River and the Chemin des Dames ridge. Along the Aisne the preponderant power of the defense over the offense was reemphasized 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. Both sides were in the process of discovering that, in lieu of frontal assaults for which neither had the manpower readily available, the only alternative was to try to overlap and envelop the other’s flank, in this case the one on the side pointing toward the North Sea and the English Channel. Thus began the “Race to the Sea,” in which the developing trench networks of both sides were quickly extended northwestward until they reached the Atlantic at a point just inside coastal Belgium, west of Ostend.

The First Battle of the Marne succeeded in pushing the Germans back for a distance of 40 to 50 miles and thus saved the capital city of Paris from capture. In this respect it was a great strategic victory, since it enabled the French to renew their confidence and to continue the war. But the great German offensive, though unsuccessful in its object of knocking France out of the war, had enabled the Germans to capture a large portion of northeastern France. The loss of this heavily industrialized region, which contained much of the country’s coal, iron, and steel production, was a serious blow to the continuation of the French war effort.

The Belgian army, meanwhile, had fallen back to the fortress city of Antwerp, which ended up behind the German lines. The Germans began a heavy bombardment of Antwerp on September 28, and Antwerp surrendered to the Germans on October 10.

After the failure of his first two attempts to turn the Germans’ western flank (one on the Somme, the other near Arras), Joffre obstinately decided to try again yet farther north with the BEF—which in any case was being moved northward from the Aisne. The BEF, accordingly, was deployed between La Bassée and Ypres, while on the left the Belgians—who had wisely declined to participate in the projected attack—continued the front along the Yser down to the Channel. Erich von Falkenhayn, however, who on September 14 had succeeded Moltke as chief of the German general staff, had foreseen what was coming and had prepared a counterplan: one of his armies, transferred from Lorraine, was to check the expected offensive, while another was to sweep down the coast and crush the attackers’ left flank. The British attack was launched from Ypres on October 19, the German thrust the next day. Though the Belgians of the Yser had been under increasing pressure for two days already, both Sir John French and Ferdinand Foch, Joffre’s deputy in the north, were slow to appreciate what was happening to their “offensive,” but in the night of October 29–30 the Belgians had to open the sluices on the Yser River to save themselves by flooding the Germans’ path down the coast. The Battle of Ypres had its worst crises on October 31 and November 11 and did not die down into trench warfare until November 22.

By the end of 1914 the casualties the French had so far sustained in the war totaled about 380,000 killed and 600,000 wounded; the Germans had lost a slightly smaller number. With the repulse of the German attempt to break through at the Battle of Ypres, the strained and exhausted armies of both sides settled down into trench warfare. The trench barrier was consolidated from the Swiss frontier to the Atlantic; the power of modern defense had triumphed over the attack, and stalemate ensued. The military history of the Western Front during the next three years was to be a story of the Allies’ attempts to break this deadlock.