Conclusion: In my opinion, the failure of the Plan was due to Germany having too much confidence and miscalculating the enemy by thinking that Belgium resistance was not strong enough to fight against them and that the Russians would be slow to mobilise its troops to war. Also the Germans did not have enough resources to keep fighting for a long period of time.
You should add more info such as when the french reserves mobilized to the marnes area of battle field the french generals ordered them to mobilize in taxis. They took over 250 taxis constantly to the frontline day and night to stop the Germans advancing on Paris.
There are six main reasons the Schlieffen Plan failed. The first reason is that, in order to invade France, the German first and second armies were in Belgium needing to get to and conquer Fort Liege. They'd expected Belgium not to fight back and allow German control but Belgium did. This delayed Germany 10 days however they still conquered Fort Liege. If Belgium hadn't resisted German forces then the Germans would've had those extra 10 days and could've used it very well to their advantage.
The second reason is that the BEF had, unbeknownst to the Schlieffen Plan, crossed the channel and fought the German first army, under the lead of Alexander von Kluck, at Mons in Belgium which held the German first army up for two days. It may have been longer if the BEF hadn't heard that the French Fifth army was retreating, so the BEF did likewise. Then they marched south 125 miles to the River Marne. If the BEF hadn't retreated, they may have either stopped Kluck entirely or the entire BEF may have fallen.
The Schlieffen Plan had anticipated, and counted on, the Russian armies taking longer to mobilise. However, the Russian Armies invaded Prussia on 17th August. Moltke sent half his right wing to the German Eighth army which left the right wing weakened and more vulnerable. Had the Russians taken longer to invade Prussia then Moltke wouldn't have sent more men to Germany's eighth army and then no other armies in the battle would've noticed a weak spot in the German defence.
Joffre now realised that the Germany was attacking most from the left and dispatched the Army of Alsace, the French Sixth army, to his extreme left that covered and protected Paris. If Joffre hadn't noticed this then he probably wouldn't have dispatched the newest army and Paris' defence would've been weaker.
The fifth reason seems very superficial but it very relevant. As the French government retreated to Bordeaux, the German First and Second armies reached then River Marne where they would have to face Belgium yet again with the six corps they had left (1 army and a half) and had lost a lot of men. However, German troops were tired and needed to rest before they fought Belgium if they wanted a chance of winning. I think that the main problem with that was that they had no idea where Belgium armies were so they didn't know if they might be attacked while they were sleeping. Kluck's and Bulow's army was short of supplies and they could not go back with the prospect of Belgium attack staring them in the face. If the German armies hadn't been tired then they could've carried on fighting Belgium and defeating them.
The French caught up with the now-rested German first and second armies and attacked. Reinforcements were rushed from Paris to the battle scene. The French forced Kluck's and Bulow's armies to separate and this opened a gap 30 miles wide. The French fifth army and the BEF entered this gap and posed a threat to the German first and second armies on both sides. Bulow felt he had no choice but to retreat and Kluck had to follow. The Schlieffen Plan had failed.
The Failure of the Schlieffen Plan
Having attacked in the West the Germans soon found their detailed plans were unravelling all around them.
The first blow was the unexpectedly strong resistance from the Belgians, both in military terms and in terms of the destruction of infrastructure. Railway lines and bridges could be replaced but the destruction of railway tunnels caused considerable hold-ups.
Soon the Schlieffen Plan was falling behind schedule, though Liege was taken only two days late. However, the delays enabled the British Expeditionary Force to land and play a part in delaying the Germans at Mons and Le Cateau.
At the same time the French Plan XVII (the advance into Lorraine) failed at the cost of 300,000 casualties. However, this allowed Marshall Joffre the time to redirect his forces in order to counter the German advance; time he might not have had if his troops had been allowed to advance into Lorraine as in Schlieffen's original plan.
On August 25th 1914 Moltke was forced to send 60,000 troops to the Eastern front to deal with the unexpected Russian invasion; given that he also had to leave two corps to deal with the Belgian army, his crucial right wing was now reduced from 16 to 11 corps.
The last day of August proved critical as the French halted the German Second Army at Guise and von Kluck wheeled south-east to assist, thereby changing the direction of his advance to the east rather than the west of Paris. The capital could not now be encircled.
In any case, the shortage of men was forcing all the German armies to close up on each other. By the time they reached the Marne they were exhausted; they were short of supplies and had only six days left in which to win and then turn east.
What was of even greater concern to the Germans was that they were now increasingly vulnerable and their right wing was now exposed to a flanking attack from the direction of Paris.
The French counter attack – the Battle of the Marne – began on September 5th. A gap appeared between Army Groups 1 and 2 and the BEF stumbled into it. All was confusion. It would appear that at this decisive moment, when a decision had to be made, there was no communication between the Army Groups 1 and 2 and Moltke for four days.
Eventually, Moltke ordered a retreat to the Aisne. However, by now he was a broken man, believing the war to be lost, and he was quietly replaced on September 14.
Whether or not the Germans had needed to retreat at this point has been debated ever since. Nevertheless the great gamble had failed and there existed no fall-back plan.
Therefore von Falkenhayn tried to resurrect the Schlieffen plan. He reinforced the right and decided on an outflanking movement. In what has been inaccurately termed the 'race to the sea', each of the two armies subsequently tried unsuccessfully to turn the flank of the other before halting at the English Channel.
This second failure shook Falkenhayn, and he informed the Kaiser on November 13th that the army was exhausted and that the campaign in the West had probably been lost.
He stated: 'As long as Russia, France, and England hold together, it will be impossible to beat them'. Chancellor Bethmann Hollweg, however, refused to contemplate defeat at this early stage and the war went on.
Now the soldiers dug in to avoid the murderous fire of the machine gun, and soon a line of trenches 475 miles long ran from Flanders to Switzerland. Static trench warfare had begun. It was a stalemate.
Conclusion
What the German generals had failed to appreciate in 1914 was that the alliances had created a remarkable balance of power in Europe: no single country had sufficient superiority for decisive victory in the short term.
There is no doubt that Germany could have beaten France on a one to one basis, just as Russia could have beaten Austria, but 1914 was not to be a rerun of 1870.
The alliance system virtually guaranteed that the war would not be decided quickly.
Once the war became one of attrition, resources would prove decisive; and the Central Powers simply didn’t have the numbers.
The Allies mobilised approximately 40 million men during the course of the war, whereas the Central Powers could only manage about 25 million. Given these odds, it is remarkable that Germany had such military success and held out for so long.
Accordingly, they suffered defeat and a complete breakdown of their economic and political structures. It was the price they paid for starting the war.
The failure of the Schlieffen plan had embroiled Germany in a long term war they simply could not win.
In a sense then the Great War was over before Christmas and Germany had lost, it would simply take four more years to confirm an outcome that had been decided in September 1914 when Schlieffen’s great plan had failed to achieve the short, decisive victory that Germany so desperately required.
