Austria-Hungary declared war with Serbia on 28th July 1914.
Germany pledging its Alliance with Austria-Hungary, the German army invaded Neutral Belgium on August 3rd 1914.
Germany had prepared itself for any future war in the early 20th century nine years earlier with the concept of " Schlieffen Plan" With Russia on the east and the French on the west both countries part of the Anglo-Russian Entente, the Germans had prepared themselves to be fighting on two fronts.
The assassination of Austria's crown prince on 28 June 1914 provided the pretext for the Austrian Empire to attack Serbia and trigger World War I. After four years of warfare, in which approximately two million German soldiers were killed, a general armistice ended the fighting on 11 November, and German troops returned home.
In the German Revolution (November 1918), Emperor Wilhelm II and all German ruling princes abdicated their positions and responsibilities.
Germany's new political leadership signed the Treaty of Versailles in 1919. In this treaty, Germany, as part of the Central Powers, accepted defeat by the Allies in one of the bloodiest conflicts of all time.
Germans perceived the treaty as humiliating and unjust and it was later seen by historians as influential in the rise of Adolf Hitler.
After the defeat in the First World War, Germany lost around 13% of its European territory (areas predominantly inhabited by ethnic Polish, French and Danish populations, which were lost following the Greater Poland Uprising, the return of Alsace-Lorraine and the Schleswig plebiscites), and all of its colonial possessions in Africa and the South Sea.