The Schlieffen plan was devised as a way to help Germany survive the earliest phase of World War I by knocking out its western opponent France before turning its attentions to Russia in the east. The plan called for an advance through Belgium and a rapid assault on Paris. The ultimate goal was to take France out before Russia could mobilize its forces.
The Schlieffen plan was sensible in theory, but it utterly failed in practice. Count Alfred von Schlieffen knew that while Russia had enormous manpower resources, its technological deficiencies meant it would take weeks before the country could reach a war footing and present a significant threat to Germany.
Fortunately for the Allies, however, he vastly underestimated the difficulty of maintaining the rapid advance necessary to knock France out of the war quickly. German forces were able to make deep inroads into French territory, and the French defenses were ill suited to thwarting the German advance. However, as the German forces moved further away from their homeland, their supply lines became longer and harder to maintain, and the Allies were able to resupply their forces much more effectively. Ultimately, the advance turned into a stalemate, leading to years of stagnant and bloody trench warfare in the west.