The Schlieffen Plan was Germany's operational doctrine at the outset of World War I. Though it nearly succeeded, the plan did not achieve its intended effect. The Schlieffen Plan was devised in 1905 as a response to the twin threats of France and Russia. It called for the swift defeat of France, which was to be followed by a transfer of troops to the Russian front before Russia could mobilize.
The plan anticipated a rapid strike against France through Belgium, whose neutrality had been guaranteed by Britain, and a six-week campaign to break the French war effort. This timetable was drafted with the assumption that Russia needed at least six weeks to mobilize a sizable fighting force and transport it to Poland. In fact, Russia mobilized somewhat faster than expected, and French resistance at the Battle of the Marne was stiffer than the diminished German force anticipated.
Ultimately, the Schlieffen Plan failed because it had only one victory condition: 100-percent success. If any part of the plan failed to work perfectly, the whole plan would come to ruin. In the event, Germany found itself bogged down in trench warfare against an unbroken France, fighting desperately against a huge Russian army and at war with Britain over its violation of Belgian neutrality.