In 1913, General Ferdinand Foch and General Joseph Joffre, developed a strategic plan for deployment of the French Army in case of war with Germany. This became known as Plan 17 and involved the invasion of Lorraine, an area that had previous been under the control of the French. Belgian neutrality was guaranteed by Britain under a treaty signed in 1839. Sir Edward Grey, Britain's foreign secretary, warns Germany that it would go to war if Belguim was attacked.
On 14th August, 1914, the French Army, led by Ferdinand Foch, Auguste Dubail and Michel Maunoury marched into Lorraine. The French soldiers were no match for the machine-guns and heavy artillery of Crown Prince Rupprecht and German Sixth Army and were forced to retreat. By 22nd August the French troops had left Germany and was back in the fortress zones of Belfort, Epinal and Toul.