World War I which was also called the "Great War" began in 1914 and lasted until 1918. There were many causes of WWI including the assassination of Franz Ferdinand, nationalism, and militarism.
The Assassination of Franz Ferdinand
On June 28, 1914 Archduke Franz Ferdinand of Austria and his wife were shot by a Serbian nationalist. The Serbians wanted to convert the Austro-Hungarian Monarchy (the Dual Monarchy) into a Triple Monarchy so that they could be included. The Serbians wanted to be self governed like Austria and Hungary. However, Austria could not find any evidence that the Serbian government had any connections with the assassination. Austria wanted to be assured of the German support just in case of war. On July 6, Germany assured Austria that if there was an Austro-Serbian war, Germany would stand by Austria and give them support as an ally. When Austria sent written demands to Serbia, they didn't like their response so they declared war on July 28, 1914.
All the countries within Europe were building large armies during the pre-war years. Before the conflict happened, the militaries of each country had drawn up complete plans for preparation. Secret battle plans brought along spies, which spread great hate and fear.
Collapse of the Alliance System
The alliances were made in secret which brought along distrust and suspicion among the European powers. Everyone was so suspicious they never talked about specific plans if a war was to break out because they feared distruction.