The Schlieffen plan was based on a theory that if Germany went to war, it would be attacked from two sides, France and Russia. But the Germans saw France as a much weaker country than Russia. The plan was devised by Alfred von Schlieffen, it was started before the twentieth century, way back in 1897.
Schlieffen thought that if France were defeated, Russia and Britain would be unwilling to carry on. He calculated the Russian army would take 6 weeks to mobilize (group), to start an attack, so it was vitally important to defeat the French quickly.
Schlieffen also took in to consideration the fact that the French had a lot of defence (I .e posts and bunkers) on the borders of Germany. The plan used 90% of Germany's army to attack France, so instead of going through France, the troops were sent through Holland and Belgium for a Scythe like' attack. The rest were sent to the defence posts.
Later Helmuth von Moltke took over as German Army chief of staff and altered the plan so Holland was not invaded. Went Belgium refused the Germans to be allowed through, they were surprised. So the Germans had to fight their way through Belgium, this slowed and tired the troops.
Then disaster happened. The Russian army only took 10 days mobilize, and were surprised how fast they had advanced into East Prussia. To make matters worse, Britain declared war on Germany for invading Germany due to the treaty of London (1839).
Te Schlieffen plan was Germanys only plan during WWI and it had failed.