Final answer:
The Balkans were known as the powder keg of Europe prior to World War I because of the long history of ethnic disputes and nationalism in the region, along with the weakening power of the Ottoman Empire and the desire of other powers to expand their influence.
Step-by-step explanation:
The Balkans were known as the powder keg of Europe prior to World War I mainly because of the long history of ethnic disputes and nationalism in the region. The Balkans were home to people of different nationalities and religions, which created tension and conflict. The weakening power of the Ottoman Empire and the desire of other powers, such as Austria-Hungary, to expand their influence in the Balkans further added fuel to the volatile situation.