Firstly, the Germans had not considered that the Belgians would resist. The Belgian army was nearly wiped out by December 1914, however they had slowed they Germans down significantly, particularly in their fortified bases in Antwerp and Liege, so that they were still in Belgium well after their 6-week deadline to be in Paris. The Belgians had also managed to hold out in the bases whilst the German army attacked France, which meant that the Germans could not afford to bring their whole army across to France, as they would be attacked from behind and their supplies would be cut off.
Another factor to add to the failure of the Schlieffen plan was the support of the British. Although the BEF was pitifully small compared to the huge conscript armies of the French and German, the soldiers were well trained and were a helpful back up for the French.
The Germans had also vastly underestimated the strength and speed of the Russian army. The Schlieffen plan was relying on a Russia that was as weak as it was in 1905, a backward nation still recovering from a rebellion. The fact was that Russia had spent the best part of 10 years rebuilding its army, upgrading infrastructure, and building a sizeable arsenal of weapons and artillery. Instead of the estimated six weeks to mobilize, two Russian armies were advancing into Germany within 10 days!
The major reason that all the nations were able to mobilize much quicker than in previous wars, and something that the Germans counted on the Russians to be lacking in, was a well-connected rail network. This allowed the vast conscript armies to congregate within weeks, rather than waiting months as people trickled into the cities.
Because the Germans were held up much more than expected, the French government were able to escape to Bordeaux in the south and continue the war effort. This prevented a quick surrender and meant the war was prolonged.
Having realised the Schlieffen plan had been a complete disaster the German commander, General Von Moltke knew the only chance was to outflank the British and the French by travelling northwest towards the coast. However, the French did the same, so they ended up in a race to the sea. The British also headed north, although their reason was to stop the German army gaining vital ports like Calais, which could then be used to stage a naval assault on Britain. If the Germans held the channel ports, it would also mean that Britain would be unable to send reinforcements and supplies to the BEF, which would have been left virtually stranded.
In order to prevent heavy losses on an open battlefield, both sides decided to dig in and build trenches whilst travelling north. Each side tried to out-dig the other, and so the line of trenches slowly snaked north, finally reaching the coast slightly north of Ypres.
The widespread use of new machinery meant that heavy casualties would be inflicted in any war. The use of machine guns and vastly improved rifles meant that soldiers were in danger of being killed or wounded at a much longer range. Compounded with the stationary position of the trenches, a few machine guns could hold a huge stretch of land, and those that did reach the other side had barbed wire to contend with. New forms of artillery also made armies much more vulnerable. To protect themselves from these dangers, the commanders simply ordered the armies to stay put and fortify and expand the trenches, making bunkers and a network of interlinking tunnels and open trenches. The trenches now stretched the length and breadth of France, and the no man’s land in between meant that any assault on the other would be suicidal.
In truth, the failure of the Schlieffen plan, the Belgian and British war efforts and new weaponry alone would not have caused such a long war. They, combined with other reasons, led to the digging of the trenches. Arguably, if both armies had not dug in and created trenches, the war would never have slowed, and the mobile battles of the 19th Century would have remained the norm. The main reason for the prolonging of the war was the instinct to dig in to prevent deaths. This changed the war from a quick war of movement to a stagnated stalemate in which neither side could deliver a knockout blow. As Crown Prince Wilhelm said after the Germans had dug in, “The war is lost. It will go on for a very long time, but it is already lost”.