That would be nationalism in the Balkans. Prior to WWI, Austria-Hungary had annexed Bosnia and Herzegovina, angering many in the Balkans especially those who wished to see a unified Yugoslavia. One such group was the Young Bosnia revolutionary organization, whose membership included Gavrilo Princip, the man who assassinated Archduke Franz Ferdinand in 1914, thus starting WWI.
World War I had many causes. Among them was the growing nationalism in many areas of the world. Peoples of the Balkan Peninsula, for example, experienced a surge of nationalism in the years before the war. Many areas of the Balkans were under the control of large empires, including the Ottoman and the Austro-Hungarian empires. The people of the Balkans believed that they should be able to govern themselves and began to fight against the rule of the controlling empires.
