It will be remembered that at the close of the battle of Montmirail on the evening of September 8, 1914, the western flank of Von Bülow's army had been exposed by the advance of General d'Espérey and the retreat of General von Kluck. Information of this reached Foch, and despite the danger of the maneuver, he thrust out his mobile left like a great tongue. That night the weather turned stormy, facilitating this move. At one o'clock in the morning, the statement has been made, word reached General Foch indirectly that air patrols had observed a gap in the alignment of the German armies between General von Bülow's left and General von Hausen's right.
During the darkness and the rain, therefore, General Foch had worked two complete surprises on General von Bülow. He had enveloped the German commander's right flank, and was safely ensconced there with General d'Espérey's army behind him, since the latter had by now advanced to Montmirail. At the same time he had thrust a wedge between Von Bülow and General von Hausen, threatening General von Bülow's left flank as well. The first was a seizure of an opportunity, executed with military promptness, the second was a bold _coup_, and its risk might well have appalled a less experienced general.
Considering the westernmost of these movements first, it will be seen at once how the enveloping action brought about the "Affair of the Marshes of St. Gond." General von Bülow's army was stretched in an arc around the marshes, which, it will be remembered, have been described as a pocket of clay, low-lying lands mainly reclaimed, but which become miry during heavy rains. It was General von Bülow's misfortune, that, on the very night that his flank was exposed, there should come a torrential downpour. These same marshes had figured more than once before in France's military history, and General Foch, as a master strategist, was determined that they should serve again. When the rain came, he thanked his lucky stars and acted on the instant.
When the morning of September 9, 1914, dawned, the left wing of General Foch's army was not only covering the exposed flank of General von Bülow's forces, but parts of it were two miles to the rear. Under the driving rain, morning broke slowly, and almost before a sodden and rain-soaked world could awake to the fact that day had come, General Foch had nipped the rear of the flank of the opposing army, and was bending the arc in upon itself. Under normal circumstances, such an action would tend but to strengthen the army thus attacked, since it brings all parts of the army into closer communication. But General Foch knew that the disadvantages of the ground would more than compensate for this, since the two horns of General von Bülow's army could not combine without crossing those marshes, now boggy enough, and growing boggier every second. The task was harder than General Foch anticipated, for the same rainy conditions that provided a pitfall for the Germans were also a manifest hindrance to the rapid execution of military maneuvers. But, in spite of all difficulties, by evening of that day, the flank broke and gave way, and two entire corps from General von Bülow's right were precipitated into the marshes. Forty guns were taken--to that time the largest capture of artillery made by the Allies--and a number of prisoners. Hundreds perished miserably, but General Foch held back his artillery from an indiscriminate slaughter of men made helpless in the slimy mud. Thus ended the "Affair of the Marshes of St. Gond," which broke still further the German right wing.