Why did the Schlieffen Plan ultimately collapse?

The Schlieffen plan was the German plan of attack designed to defeat France quickly and then turn on Russia. It was intended to avoid a war on two fronts. However, the plan was based on a number of assumptions, and these turned out to be flawed.

1. It was thought that Russia would take at least six weeks to mobilize. In reality Russia mobilized in just ten days.
2. The German General Staff thought that Belgium would not resist any Germany attack. When Germany invaded on 2nd August 1914, they were held up by the Belgium army, backed up by the BEF (British Expeditionary Force).
3. It was also estimated that France could easily be defeated in six weeks. Backed by the British, the French managed to halt the German advance.
4. The Germans thought that Britain would not fight and that its army was just a huge joke.

With the quick Russian mobilization, Germany was forced to withdraw troops from France defend her eastern border.

Thus, the Schlieffen Plan failed because it was based on a number of monumemtally arrogant assumptions that were flawed. For years the German General Staff had been badly out of touch with reality.