In the end of 19th century, the idea of nationalism changed its nature and became closely associated with imperialism and fanaticism that finally resulted in the 1st world war.
After 1871, the first evidence of such nationalism in Europe emerged in Balkan States– Greece, Bulgaria, Romania, Serbia, Bosnia-Herzegovina, Macedonia etc.
The people of this region were called Slavs, an ethnic group in Eastern Europe.
Most of them were at that time were under Ottoman Empire. But the rise of nationalist sentiment in West Europe inspired them to be independent and unite into nations based on common history and culture.
This way, the south-east part of Europe, called the Balkan region, became a region of turmoil. One after another, independent nations emerged out of Ottoman empire.
However, the nations of West Europe helped in growing nationalism in Balkan States for their own motives. They had their imperialist goals and agendas there.
Therefore, Balkan region became a great game among other big powers like Britain, Russia, Germany and Austro-Hungary.
The Balkan States had internal rivalries and intense competition to capture each other’s territory and expand themselves.
All these factors contributed to the outbreak of WWI.
Nationalism can end Imperialism
Apart from European context, from the beginning of 20th century, nationalism played a major role in ending the European empires- British Empire, French Empire etc.
The growth of nationalism in the colonies of the empires in Africa and Asia led to Wars of independence, revolutions and mass-movements.
Therefore, in the second half of 20th century, we see a period of decolonization and emergence many new nations which were once under European occupation.
The nationalist sentiment remained the prime driving force for their independence.