On the Western Front, hostilities commenced 2 August 1914 with the German invasion of Luxembourg and Belgium, designed to envelop Paris in a right sweep that avoided the most heavily defended French territory. The French, whose main effort was directed towards Alsace and Lorraine as a prelude to an invasion of Germany, were thrown onto the defensive on their left flank, which included one cavalry and four infantry divisions of the British Expeditionary Force. At the end of a two-week retreat from Mons the French managed to stop the Germans on the outskirts of Paris in the First Battle of the Marne. German forces retreated to the River Aisne and dug in, the first act of trench warfare that was to dominate the war.