The Schlieffen Plan called for an attack through Belgium. The Germans were to avoid the main French fortifications, on the German frontier. They invaded Belgium on August 3rd 1914. The Germans pushed through the Belgium army with little difficulty. After destroying the main fortifications in Liege and Namur, the German First Army and Second Army under the command of Alexander von Kluck and Karl von Bülow, moved toward Paris. At the Battle of Mons the British Expeditionary Force (BEF) halted the German forces temporarily. They inflicted heavy casualties on the Germans. But the French army and the BEF finally retreated towards the River Marne where they waited for the Germans. With no real opposition the Germans moved as quickly as possible and prepared to face the French and British at the Marne River. 