Final answer:
The Schlieffen Plan failed largely due to the underestimation of Belgian and French resistance, unanticipated British involvement, and the quicker than expected Russian mobilization, leading to a prolonged and costly trench warfare.
Step-by-step explanation:
The Schlieffen Plan was a German war plan that outlined a strategy for a quick victory against France by moving through Belgium before having to face Russia on the eastern front. This plan was developed prior to World War I but it failed for several reasons. Firstly, the German military underestimated the resistance of the Belgian and French forces. Secondly, Britain's involvement was not anticipated as Germany assumed Britain would refrain from sending its army to defend France.
Lastly, the speed of Russian mobilization was greater than anticipated, forcing Germany to fight on two fronts simultaneously, which was the exact scenario the Schlieffen Plan aimed to avoid. These factors culminated in stalemate and trench warfare, with soldiers enduring harsh conditions over the subsequent years.