The correct answer is true.
It is true that Imperialism and Nationalism were often opposing forces that increased tensions in Europe.
There were many causes that accelerated the beginning of World War 1. Of course, Imperialistic ideas was one of the major reasons as well as nationalistic principles that made nations to oppose the advancement of imperialistic objectives of powerful European nations. Ethnic problems also was a major cause of the war, as happened in the Balkans. What started World War 1 was the assassination of Austrian Archduke Franz Ferdinand and his wife in Sarajevo, in 1914.
