The Schlieffen Plan failed because of bravery of BEF In 1914, Germany would go to war with Russia. If this happened then Germany assumed french would also attack them as they were friends of Russia. This meant that German would be attacked on both sides of the country, so Germany wanted to avoid spreading the army into two to defeat France quickly and then attack Russia. The German Army Chief of Staff, Alfred von Schlieffen was asked to plan a war on two sides. He think that it was important to get a fast break attack.
Schlieffen wanted to attack France through Holland, Belgium and Luxembourg. He planned to use 90% of the German army to deliver a knock out blow to France. How ever in the battle of liege in Belgium, the local army helped by the British Expeditionary force, which arrive very quickly. And defending the Germen and at the same time, Russian prepared their army to attack. Makes the plan failed. The main reason of Schlieffen plan failed was the british expeditionary force helping out.
This is a surprise defending to the Germen and had been shocked. Makes the plan totally went to the wrong side. Also, at the same time the Russian was ready to attack Germany at the other side, and forced the attacking troops need to be back and ready for defense. The Germen advance was slowed down. Through, the helping from the British to the Belgium, the Germany got another enemy to fight in further days. Therefore the Germany will be much more difficult to be the winner of the war.