Final answer:
The unexpected prompt arrival of the British Expeditionary Force (BEF) and the quick Russian mobilization thwarted the Schlieffen Plan as Germany had to fight on two fronts and could not swiftly defeat France. This delay allowed the Allies time to counter-attack, leading to a prolonged and destructive stalemate in the trenches of World War I.
Step-by-step explanation:
The Schlieffen Plan was a strategic military operation developed by Germany with the intent to quickly defeat France by advancing through Belgium, and subsequently turn to counter the Russian threat in the east. This military strategy failed for several reasons. One critical reason was the unexpected timing and arrival of the British Expeditionary Force (BEF) to assist France. The German military planners did not anticipate Britain entering the war so promptly in defense of Belgium and France, nor did they expect the BEF to be effective so soon. Moreover, the German assumption that Russia would require more time to mobilize proved incorrect as Russian forces mobilized and launched their offensive much earlier than expected.
The Germans faced a war on two fronts, which the Schlieffen Plan aimed to avoid. The presence of the BEF along with a robust French resistance, compounded with the need for German forces to be diverted to the East to respond to the Russian mobilization, significantly diluted the strength and speed of the German offensive in the West. This resulted in Germany not securing Belgium until late August 1914, further delaying their advance into France and allowing the Allies time to regroup and mount a counter-offensive.
The simultaneous underestimation of both the BEF and Russian forces led to imbalances in German troop deployments and allocated resources, contributing to the failure of the Schlieffen Plan and the subsequent prolonged and bloody trench warfare that characterized World War I.