Imperialism increased tensions in Europe because

European Expansionism ... The expansion of European nations as empires (also known as imperialism) can be seen as a key cause of World War I, because as countries like Britain and France expanded their empires, it resulted in increased tensions among European countries.

European colonies competed for colonies in Asia and Africa. the belief or desire of a government or people that a country should maintain a strong military capability and be prepared to use it aggressively to defend or promote national interests.