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War plans – Battle of the Frontiers

The Western Front was the place where the most powerful military forces in Europe, the German and French armies, met and where the First World War was decided. At the outbreak of the war, the German Army, with seven field armies in the west and one in the east, executed a modified version of the Schlieffen Plan, bypassing French defenses along the common border by moving quickly through neutral Belgium, and then turning southwards to attack France and attempt to encircle the French Army and trap it on the German border. Belgian neutrality had been guaranteed by Britain under the Treaty of London, 1839; this caused Britain to join the war at the expiration of its ultimatum at midnight on 4 August. Armies under German generals Alexander von Kluck and Karl von Bülow attacked Belgium on 4 August 1914. Luxembourg had been occupied without opposition on 2 August. The first battle in Belgium was the Battle of Liège, a siege that lasted from 5–16 August. Liège was well fortified and surprised the German Army under Bülow with its level of resistance. German heavy artillery was able to demolish the main forts within a few days. Following the fall of Liège, most of the Belgian field army retreated to Antwerp, leaving the garrison of Namur isolated, with the Belgian capital, Brussels, falling to the Germans on 20 August. Although the German army bypassed Antwerp, it remained a threat to their flank. Another siege followed at Namur, lasting from about 20–23 August.

The French deployed five armies on the frontier. The French Plan XVII was intended to bring about the capture of Alsace–Lorraine. On 7 August, the VII Corps attacked Alsace to capture Mulhouse and Colmar. The main offensive was launched on 14 August with the First and Second Armies attacking toward Sarrebourg-Morhange in Lorraine. In keeping with the Schlieffen Plan, the Germans withdrew slowly while inflicting severe losses upon the French. The French Third and Fourth Armies advanced toward the Saar and attempted to capture Saarburg, attacking Briey and Neufchateau but were repulsed. The French VII Corps captured Mulhouse after a brief engagement first on 7 August, and then again on 23 August, but German reserve forces engaged them in the Battle of Mulhouse and forced the French to retreat twice.

The German Army swept through Belgium, executing civilians and razing villages. The application of "collective responsibility" against a civilian population further galvanised the allies. Newspapers condemned the German invasion, violence against civilians and destruction of property, which became known as the " Rape of Belgium." After marching through Belgium, Luxembourg and the Ardennes, the Germans advanced into northern France in late August, where they met the French Army, under Joseph Joffre, and the divisions of the British Expeditionary Force under Field Marshal Sir John French. A series of engagements known as the Battle of the Frontiers ensued, which included the Battle of Charleroi and the Battle of Mons. In the former battle the French Fifth Army was almost destroyed by the German 2nd and 3rd Armies and the latter delayed the German advance by a day. A general Allied retreat followed, resulting in more clashes at the Battle of Le Cateau, the Siege of Maubeuge and the Battle of St. Quentin (also called the First Battle of Guise).