Ethnic tensions in Central and Eastern Europe were caused by the tendency of different groups to have their own nation-states.
During the XIX and early XX centuries, nationalism was a key feature of the European ideological landscape. According to that ideology, every nation (defined as a group of people that shared a common language, history, customs, etc) had to have their own political organizations in the form of nation-states. Nationalism was one of the main reasons behind the start of WWI, which happened in The Balkans, after a Serbian Nationalist killed the heir to the Austro-Hungarian crown. The assassination was motivated by the desire of Serbian nationalists to separate from the Austro-Hungarian Empire and form their own State.
