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 Siege of Liège, which 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.