Which of the Causes of World War 1 (Nationalism, Imperialism, Militarism, Alliances) do you believe to be the most significant and why (5-7 sentences minimum) ?

Nationalism

Explanation:

The nationalism in Europe by the beginning of the XX century was strong and a major force in the "separatist" movements in the Balkans, where the former Austro Hungarian Empire wanted to continue to seize control of Slavic nations that were oppressed.

Many of these peoples felt that they belonged to a bigger group "the Slavs" and they were protected by the pan-Slavic nationalists, that directly promoted the killing of Archduke Franz Ferdinand in a defiant act to one of the greatest empires of that time.

The Balkans would continue to be a fierce zone where many new nationalist movements would continue to cause outbreaks of conflicts even to our present day.