The First Battle of the Aisne (13-28 September 1914) marked the end of mobile warfare on the Western Front and the start of the period of static trench warfare that would last until 1918 (First World War). On 11 September, having been defeated in the First Battle of the Marne (3-9 September), Hermuth von Moltke, the Chief of the German General Staff, issued orders to retreat to the line of the Aisne and to fortify the high ground north of the river. This was his last act as Chief of the General Staff – on 14 September he was replaced by Erich von Falkenhayn. This was the same day that saw the German 1st and 2nd armies take up their positions on the Aisne and begin to dig in.
The new German line was an ideal defensive position. Any allied attack would have to cross the River Aisne and then attack up a 500 foot high ridge. On top of that ridge was the Chemin des Dames, a road that gave the Germans an easy way to move troops along the top of the hills. The German army had practised entrenching manoeuvres before the war, and quickly dug itself in with Kluck’s First Army to the west and von Bülow’s Second Army to the east.
The battle of the Aisne began before the Germans had reached their new positions on the ridge. On 12 September the British 11th Infantry Brigade had reached the crest of the high ground at Venizel, in the middle of the Aisne line, but the BEF soon became bogged down in the centre of the line. On 13 September the French Sixth Army had attempted to get around the western flank of the Chemin des Dames ridge near Compiègne, but had been stopped by German resistance.
The best chance for the allies came on the right of their line, where the French Fifth Army found a gap between the two German armies, and reached Berry-au-Bac, on the northern side of the river, but the gap was closed by the arrival of the German Seventh Army under General Josias von Heeringen.
The battle continued until the end of September, but it quickly became a side issue. Both sides attempted to use the fighting on the Aisne to pin their opponents in place, while their remaining mobile armies took part in the Race to the Sea. This saw both sides attempt to find an open flank which would allow them to outflank the enemy. By the time the Race to the Sea and the first battle of Ypres came to an end, the Western Front had taken shape – a 475 mile long line of fortifications running from the North Sea to the Swiss border.