Much before the war started, Germany had formulated Schlieffen Plan first in 1905 and modified it in 1911. The plan had assumed that if there was any outbreak of war in Europe, France would automatically join Russia and Germany would have to fight both of them in Western and Eastern Front. The plan had therefore proposed that the bulk of German forces were to be sent by train to Belgium and through Belgium to attack France, which was to be knocked out within first six weeks.
After defeat of France, German forces were to be switched across to face Russia, whose army was supposed to be ready only after 6 weeks. The plan had also assumed that as France was defeated within first 6 weeks, Great Britain would not be able to land their armies on French territory and will not be able to help France. Thus Germany could win the war on both Western and Eastern Front within 3-4 months. In actual practice Schlieffen Plan could not be carried out successfully.
When Germany invaded France through Belgium, its armies took more than 2 weeks to pass over Belgium and reach the border of France due to very strong resistance from Belgian Army. This delay gave enough time to Britain to land its armies on West Cost of France. Germany had planned under Schlieffen Plan to march to Paris both from Eastern and Western sides but due to British Forces on the Western Side, Germany could move towards Paris only though the Eastern Side.
Although Germany made good progress initially and its army reached within 20 miles from Paris, it could not capture Paris because they faced joined armies of France and Britain at Marne. The battle at Marne was very crucial because it halted the advance of German army. The German Army was tired due to August heat and they were short of supplies of food and ammunition. They were driven back to river Aisne. The battle at Marne was very decisive as it dashed all hopes of Germany to defeat France within first 6 weeks.
There was another very important battle in 1914 which was fought at Ypres in Belgium. The armies of France and Great Britain were able to halt the advance of Germany by holding on to Ypres in Belgium. This kept the West Cost of France safe for the armies of Britain to land and send other help. Furthermore the armies of France and Britain could prepare the trenches all along the border of France from West to East, which made it difficult for German Forces to advance. Thus the war of Ypres was another reason which caused the failure of Schlieffen Plan to finish the war before Christmas of 1914.
The British and French navies were also active against Germany. The purpose of the navies of the allied forces were to block the Germans and prevented any goods from entering or leaving German territories. At the same time they kept routes open for Britain and France to receive supplies and transport troops to the France and keep then supplied via the English Channel Ports. The British navy was successful in achieving this objective.
The main lesson learnt about warfare was how to conduct the battle from trenches. Both sides dug up huge trenches from where the soldiers were to fire at the enemy. There were barbed wires in front of the trenches to protect the soldiers. These trenches made the advance of any army very difficult as there were large number of casualties due to enemy fire from the trenches. Even if the first line of trenches was won over there was a second line of trenches which continued to fire at the enemy. At this line tanks and machine guns were also employed to block the advance of the invading army. The Germans also used poisonous gas which made British large number of British soldiers blind. But a number of times the poisonous gas blew over their own soldiers due to wind. Because of these tactics there were large number of casualties numbering about half a million in every large battle.
The morale of the French soldiers was very high as they were protecting Paris from the invading Germans. Ordinary French people also helped the French army in various methods to resist the German advance. On the other hand Germen soldiers were feeling tired and exhausted. They were fighting away from their homes and they were short of ammunition and food supplies.
The war didnt finish by Christmas 1914 because France and Belgium put up resistance to protect their countries. This gave enough time to Britain come to help France. French people resisted the advance of Germany with the help of Britain Army because of trench warfare and the usage of guns and tanks. Russia also started war on the eastern front and Germany had to face the armies on both the fronts and could not win the war as per the Schlieffen Plan.