Why didn't the war plans of Germany (the Schlieffen Plan) and France (Plan XVII) work as intended once hostilities began in 1914?

The Schlieffen Plan and Plan XVII did not work as intended due to several reasons:

1. Schlieffen Plan (Germany): The Schlieffen Plan was based on the assumption that Russia would take at least six weeks to mobilize its forces. However, Russia mobilized much quicker than expected, forcing Germany to divert troops to the Eastern Front.

2. The plan also assumed that Belgium would not resist German forces, but Belgium put up a strong fight, slowing down the German advance.

3. The plan relied on rapid movement, but the speed of the German advance was hindered by logistical problems, including supply shortages and poor communication.

4. The British Expeditionary Force (BEF) and the French army put up a stronger resistance than expected at the Battle of the Marne, further disrupting the Schlieffen Plan.

5. Plan XVII (France): Plan XVII assumed that the main German attack would come through Alsace-Lorraine. However, the main thrust of the German attack came through Belgium, catching the French off guard.

6. The plan was based on offensive tactics, but the French army was not adequately prepared for the realities of modern warfare, including the use of machine guns and heavy artillery.

7. The French also underestimated the strength of the German forces, leading to heavy losses in the initial stages of the war.

8. Both plans failed to take into account the impact of new technologies, such as machine guns and barbed wire, which made defensive positions much stronger than in previous wars.

In conclusion, both the Schlieffen Plan and Plan XVII were based on flawed assumptions and failed to adapt to the realities of modern warfare, leading to their failure once hostilities began in 1914.