Due to the Assassination of Archduke Franz Ferdinand, Militarism, Nationalism, Imperialism, and Alliances. Which made a big war happen between multiple countries.
Nationalism
The building up of a country's military.
Imperialism
I own all of this land, you will all be under my control!
When two or more countries to protect each others interest.
Assassination
The love of ones self country.
When a stronger nation or country attempting to create empires by dominating weaker nations.