The Schlieffen plan failed because Germans underestimated Russia and the plan depended on rapid deployment, which was resisted by Belgium. This plan was designed by General Count Alfred von Schlieffen in December, 1905, with the aim of defeating France and Russia. The execution of this plan compelled Britain to declare war on Germany in 1914.
In 1914, Germans believed in a possible war with Russia. If the war happened, France would come in to support Russia because they were allies. This implied that Germany would face attacks on both sides of her borders. To prevent such a situation from occurring, Germany had to create a plan. The plan was to defeat France quickly, then attack Russia, because they knew that Russia would take a long time to mobilize troops.
According the Schlieffen Plan, German had 6 weeks to secure victory against France before fighting Russia. On August 2, 1914, Germany attacked Belgium. However, the Belgian forces, assisted by the British Expeditionary Force, was able to hold back the Germany. Russian troops mobilized within 10 days, forcing Germany to withdraw her troops and concentrate on the eastern border. On September 5, 1914, France stopped the advancement of Germany, leading to the failure of the plan.