Also, the Russians surprised the Germans by attacking in the east; Moltke then weakened the right wing more by sending troops to fight the Russians. Another reason was that troops marched more than 30km every day, and, by early September were exhausted. German communication lines became extended and ground to a halt, roads were clogged with refugees and telephone lines (due to sabotage) were out and so Moltke and his staff were stuck in Luxembourg, not knowing what was going on with his army.
The Schlieffen plan was failing miserably and the German troops morale was just as miserable but still, the German army advanced until they were just 40km from Paris. Blocking their way, however, assembled by the River Marne were the British and French troops.
The reasons why the French were unable to defeat the Germans completely at the Battle of the Marne were numerous; one of the reasons was that no matter how ill spirited the German armies were, they were still large and well prepared. The French and British troops however had almost been “thrown” together and in comparison to the vast amounts of German soldiers, the British and French had an exceedingly small army.
The German armies also had far more munitions than the British and French. Joffre did however manage to form troops from the east to Paris, yet they were weak and tired and so did not really help a considerable amount. Both sides started to dig themselves in, making defensive positions by building trenches; these in turn formed the stale mate. The last reason the French were unable to defeat the Germans completely was that both sides of the war did not have a definite plan or strategy, the Schlieffen plan had failed and France simply did not know what was going on.
After the Schlieffen Plan’s failure tactics changed, both sides (France and Britain vs. Germany) had a “race” to try and get round the back of each other to attack from more stable and secure positions. Neither managed to do this and so armies ended up marching all the way to the Belgian coast, where the war stayed for the next four years, their positions hardly changing and building more and more trenches.
Due to both French and British troops, and German troops “burying” themselves into their defensive positions in their trenches, defending positions was far easier than attacking and this was another addition to the reason of the stalemate. When attacking you had to leave the safety, protection and concealment of the trenches, you were out in the open and comparatively defenceless as all that faced you was “no-man’s-land” and you knew that beyond there was hundreds of opposing troops ready to blow you down with their machine guns, and so attacking became suicidal.
Staying in your trench was by far the better option; you were hidden and highly armed. Communications were high and so trenches were supplied adequately and defence was mechanised, reinforcements and food could arrive by rail at anytime.
So, due to all these reasons, and in particular the last one, a strong, fortified stalemate developed on the Western Front, and seemingly nothing could break it.