The war on the Western Front in World War One began with the German invasion of Belgium, a stipulation of the Schlieffen Plan. Constructed by Field Marshal Alfred von Schlieffen in 1906, the Plan outlined the stages of an offensive against France.
Desperate to avoid fighting on two fronts, against both the French and Russia, the Schlieffen Plan envisaged a swift 6-week campaign against the former to allow for the focus of forces against the latter.
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The initial attack
German forces attacked through Belgium and pressed into France. Having clashed first with the French, on 23 August the German right encountered the 68,000 men of the British Expeditionary Force.
The Anglo-French forces fought the Germans to a standstill but it soon became apparent they were in severe danger of being overwhelmed by weight of numbers and retreated towards Paris. German commander Alexander Von Kluck held off at first, choosing instead to make good the losses inflicted on his force at Mons.
When he did pursue the Allies, he caused almost 8,000 casualties among the British rear-guard at the Battle of Le Cateau on 26 August.
Aerial photograph of World War One trenches on the Western Front.
Saving Paris
During the BEFs exhausting retreat to the River Marne, a distance of some 250 miles, the tiny British force remained in contact with both the French and enemy forces. Discipline and courage saved the BEF from total annihilation.
As the British retreated southwards, the Germans followed, leading them away from Paris. They had been denied the rapid capture of the capital, a key stipulation of the Schlieffen Plan.
German military planning had faltered.
The exhausted Allies turned to face the Germans at the River Marne in front of Paris on 6 September 1914. By the time the battle ended, on 12 September, the Allies had successfully pushed the Germans back across the river. Both sides were exhausted and had incurred huge casualties.
But Paris was saved and German military planning had faltered.
A French trench in north-eastern France. Credit: Library of Congress / Commons.
The German retreat
In the wake of the Battle of the Marne in September 1914, the Germans were forced to retreat to the River Aisne.
Helmuth von Moltke, commander in chief of the German army, was replaced, his nerves shot by the strain of command. His replacement, Erich von Falkenhayn, halted the German retreat and ordered that they take up defensive positions on the ridge overlooking the the river.
Falkenhayn ordered that his forces hold the territory they occupied in France and Belgium. Therefore, on 14 September, he gave the command to dig in.
The Allies, realising the German retreat was over, recognised they could not break through this line, which was defended by large numbers of machine guns. They also began to dig trenches. 
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Advances in trench building
At this stage, neither was equipped for trench warfare. Early trenches were often shallow and ill-suited to long-term habitation. British commander Sir John French was fond of saying that in these conditions, “a spade was as useful as a rifle”.
Individual trenches were slowly expanded into gargantuan trench networks with underground barracks and supply stores.
Soldiers complained that this kind of warfare was more strenuous than earlier mobile battles. A battle in the open would generally only last for a day or so, trench battles went on for several days inflicting relentless stress and fatigue.
The swift turnarounds of victory and defeat, typical of the early battles of movement, were over.
