The correct answer to this open question is the following.
The Schlieffen plan to quickly defeat France was an idea that belonged to Alfred von Schlieffen in 1905. He considered that Germany could win a quick war with France when battling Russia. The successor of von Schlieffen, Helmuth von Moltke decided to implement the plan during World War 1, but he reduced the size of the German Army and the plan completely failed. This action provoked Britain to enter the war to support France. So the plan's failure affected the participants and conditions of World War 1.