Traditional economic system of the colonies : 

The European nations vied with each other in establishing colonies. This competition led to conflicts among them and caused mutual distrust and enmity. This further drove the nations to the signing of military alliances. The Triple Alliance and Triple Entente were examples of such military alliances.

Germany, Austria Hungary and Italy were members of triple alliance.Triple entente consisted of England, France and Russia. The formation of such alliances created a war atmosphere in Europe. They started manufacturing and buying lethal weapons.

Aggressive Nationalism : 

The European nations used aggressive nationalism to invade other countries. Aggressive nationalists considered their own nations as the supreme and justified whatever be the actions of their nations. Various movements in europe were an offshoot of Aggressive nationalism.

Assassination of Francis Ferdinand : 

When Austria laid its claim over the Balkan region with the support of Germany, Serbia claimed this region with the support of Russia. During this time a Serbian youth Gaverilo Prinsep assassinated Francis Ferdinand the heir to the throne of Austria at the Bosnian capital Sarajevo in June 1914.

Austria held Serbia responsible for it and declared war on serbiaon 28 July 1914. Almost all nations of the world directly or indirectly were involved in it. Therefore this war is known as the First world war.