In 1914, World War One emerged in Europe between the Entente and Triple Alliance. These alliances covered most of Europe, and defeating the German Empire was the key to victory for the Entente side. For undoubtedly, if the Entente defeated Germany the rest of its alliance would crumble fairly quickly. Germany was defeated in 1918, and there were a variety of factors behind this eventual outcome of the war.
Before the war had even begun, the seeds for Germany's defeat had been sewn. Germany had drafted the Schlieffen Plan that became the cornerstone of its military strategy. In the event of a war in Europe, in which Germany would most likely be at war with both France and Russia, the German plan targeted a quick defeat of France. With the French then defeated, only the one front with Russia would remain; and then Germany could place its whole army on the eastern front to defeat the Russians with.
The failure of this plan to ensure a quick victory for Germany's army was one of the war's turning points. However, it was not necessarily enough to ensure Germany's defeat in the war. Although the Germans had no clear plan b, they still had one of Europe's best armies and a large navy.
Perhaps more significant was that Germany was at a numerical disadvantage. The combined Entente armies, which included the Russian army that was the largest in the world, had superiority in numbers. The Italians and then the USA also joined the Entente in 1918. As such, in the long run this superiority in numbers would wear down the Germans and left their eventual defeat more likely.
Entente naval superiority at sea was another factor in Germany's eventual defeat. The Royal Navy effectively blockaded Germany during the war, which reduced supplies its army. The U-boat campaign of 1917 was also one of the turning points in the conflict as the U-boats failed to defeat Britain. This campaign also ended U.S. neutrality, and the Americans joined the Entente's side, which further tipped the balance in favor of the Entente.
Germany lost World War One because its Schlieffen Plan was ineffective and did not defeat France quickly as expected. As a result, a war on two fronts emerged in which Germany was at a numerical disadvantage. Even though the Russians were still defeated, the eventual entry of the USA ensured that the Entente maintained a significant numerical advantage. Such superiority in numbers proved decisive for the Entente, especially in 1918 as the Germans retreated. After the Battle of Amiens, the German High Command was all too aware that Germany could no longer win the war; so the armistice was accepted.