In the first half of the nineteenth century, liberals believed that if European states were organized along national lines, these states would work together and create a peaceful Europe. They were wrong.
The system of nation-states that emerged in Europe in the last half of the nineteenth century led not to cooperation but to competition. Rivalries over colonies and trade grew during an age of frenzied nationalism and imperialist expansion. This caused the war to start because of the mass competition in trade and gaining colonies. It was also another big part because not all the ethnic groups had become nations.

The Rise of Nationalism