Schlieffen was a German general. He came up with a plan to defeat the Allied Powers. Seeing the tensions in Europe, German war planners knew that they had to come up with a plan for a preventive war. Many in Germany and Austria-Hungary feared Russia and France as future enemies. After Russia lost the Russo-Japanese war in 1905, they were eliminated as a serious threat in Europe. This is when the Schlieffen Plan came together.$^8$
The plan was to unleash the war while Russia was weak. German planners predicted that Russia will become nearly invincible in the near future. Britain and France held the same fear, but approached the situation by developing friendlier ties with Russia. The strength of the Britain France entente was tested with the First Moroccan Crisis. This showed Germany that Britain will, almost certainly, become involved in any future conflict.$^8$
In the event of a war, Germany and its allies will be almost completely surrounded by enemy alliances. As such, it was an unescapable reality that they would have to fight a 2-front war. To deal with this, Schlieffen determined that Germany’s best course of action would be break up the war into two, one front wars. It would swiftly attack France while maintaining defensive positions on its eastern boarder, and then it would turn and fight Russia.$^8$
In order to overcome the heavily defensed Franco-German border, Schlieffen had the German army invade France through Belgium and Luxemburg. Luxemburg did not have an army at all, and Germany expected the weak Belgium army to retreat.$^8$
In short, it relied on four things. One, the speed of the German army. Two, the mobilisation of the Russian army would be slow. Three, Britain staying out of the war. Four, the Belgian army retreating.$^9$
Reason one. It overestimated the distances that the Germany army would have to cover in the strict schedule. Many troops, especially those in the extreme right, were exhausted after weeks of marching. They were unable to keep up the same pace.$^9$
Reason two. After Schlieffen, Von Moltke took over the plan. He changed the distribution of troops. He placed more troops in the left flanks and sent some battalions to the eastern front. This weakened the right flank and resulted them in not being able to fully push forward and surround paris.$^9$
Reason three. The Belgian resistance was stronger than expected.$^9$
Reason four. The British did not stay out of it. They sent an expeditionary force of 80,000 troops to Belgium. They managed to inflict heavy causalities on the German troops.$^9$
Reason five. The Belgian and British forces slowed down the German advance and forced the German right flank to turn south. This gave France the time to rush troops into Paris. In the ensuing battle, Germany was forced to retreat to the River Aisne.$^9$