September 5.—General movement of the German armies across the Marne. The troops of von Kluck crossed at Trilport, Sommery, and La Ferté-sous-Jouarre; those of von Bülow at Château-Thierry; those of von Hausen at Epernay, and Duke Albert's at Chalons. Simultaneously columns of von Kluck's 2nd and 4th Reserve corps began to cross the Ourcq.
From the Marne the Germans pushed on without delay to the south. The 3rd, 4th, and 7th corps of von Kluck's army were on the march diagonally across the British near Coulommiers. They were making for La Ferté Gaucher. In face of this advance the 5th French army fell back on the latter place. This move lengthened the German flank and laid it more completely open to a British attack.
September 6.—General Joffre gave orders for a general advance. Before daybreak the 6th French, British, and 5th French armies began a combined offensive. While the 6th French army advanced eastward towards the line of the Ourcq, the British advanced north-east to the line of the Grand Morin, and the 5th French army north from east of La Ferté Gaucher upon Montmirail.
The 6th French army, driving in the German advance posts, reached Nanteuil.
The British fell upon the flank of the divisions of von Kluck's army still crossing the Grand Morin, and drove them back upon the Petit Morin.
By this unexpected and swiftly delivered blow von Kluck's army, extending from the Marne to La Ferté Gaucher, was cut into two parts.
Coincidently with the British advance the 5th French army had, in a night attack and at the point of the bayonet, driven the leading German divisions out of three villages near La Ferté Gaucher, where they had bivouacked.
In view of these attacks General von Kluck had no alternative save to retreat. To escape the British he fell back on the Petit Morin in the direction of Montmirail.
His retreat was assisted by the right of von Bülow's army, and covered by his divisions of cavalry, reinforced by von Bülow's cavalry of the Prussian Guard. The German cavalry, attacked by the French and British, was cut up with heavy loss. More than 60,000 horsemen were engaged in this gigantic combat.
September 7.—To assist the retreat, the centre divisions of von Kluck's army opposing the British made a stand upon the Petit Morin, and the army of von Bülow a stand from Montmirail to Le Petit Sompius. Along that line the 5th French army was all day heavily engaged against the left wing of von Kluck's army and the right of von Bülow's.