De Castelnau’s brilliant tactics brought him renown and the direction of the 2nd Army on the Compiègne-Arras line. Along this Front rising rectangularly from the Aisne to the north, occurred those terrific battles, which marked the historic "race to the sea." Smarting from their defeat on the Marne, the Germans sought to turn the Allies’ left; Joffre had a similar idea in wishing to envelop the enemy’s right. The resultant contest was the _course à la mer_. But the forces given to de Castelnau were inadequate for the purpose, and de Maud’huy’s army was added to the line now creeping forward like a gigantic snake to the sea. But before the Germans tried to pierce at Lens and Arras, they assailed the lower line held by de Castelnau, and the angle formed by Aisne and Oise proved a particularly warm corner. In that late September and early October, de Castelnau lost as many men as in Lorraine, and the battles, if less renowned, were as fiercely fought as those to the north of Arras on the Yser and at Ypres. The Germans in extending their lines realised that if they could reach Dunkirk and Calais they would not only cut England’s communications, but point a pistol at her heart. And so they brought up army after army until the line stretched in a solid trunk of trenches with branches towards the sea, and 800,000 Germans (eighteen Army Corps and four Cavalry Corps) made persistent efforts to break through the Allies or envelop them. To defeat this plan, Joffre formed three new armies, of which de Castelnau’s was one, and brought up the English from the Aisne, and the Belgians from Antwerp. The situation was often critical, for de Castelnau, like the other commanders, lacked ammunition. But his gift of prevision and his infinite resource saved the day. Eventually, the chief fighting was transferred to the northern part of the line, but de Castelnau’s early resistance had rendered the greatest service. And so Joffre thought, for de Castelnau was given dominion over four armies from Soissons to Verdun, the longest front in the possession of a single commander, though Foch and Dubail were also given groups of armies. His tenure of the line was distinguished for the great offensive in Champagne, whereby 23,000 prisoners and 120 guns were captured by the French, and 3,000 prisoners and 25 guns by the English in Artois. That latter feat will be ever remembered for the house-to-house fighting in Loos and for the brilliant capture of Hill 70.