On 3rd August, 1914, German troops crossed the Belgian border in the narrow gap between Holland and France. The German First and Second Armies swept aside the small Belgian Army and by 20th August had occupied Brussels.
The French commander-in-chief, Joseph Joffre, ordered his Fifth Army and the British Expeditionary Force (BEF) to meet the German advance. The German defeated the French at the battles of Sambre (22nd August) and Mons (23rd August). By the end of August the Allied armies were in retreat and General Alexander von Kluck and the German First Army began to head for Paris. What was left of the French Army and the BEF crossed the River Marne on 2nd September.
Joffre ordered a counter-attack which resulted in the Battle of the Marne (4th to 10th September). Unable to break through to Paris, the German army was given orders to retreat to the River Aisne. The German commander, General Erich von Falkenhayn, decided that his troops must hold onto those parts of France and Belgium that Germany still occupied. Falkenhayn ordered his men to dig trenches that would provide them with protection from the advancing French and British troops.
The Allies soon realised that they could not break through this line and they also began to dig trenches. After a few months these trenches had spread from the North Sea to the Swiss Frontier. For the next three years neither side advanced more than a few miles along this line that became known as the Western Front.