Yes, aggressive nationalism laden with imperialism was the main cause of the First World War. The war was cause because the leading nations of Europe all sought to aggressively defend their national interests in Europe. A part of the aggressive nationalistic posturing was the search of imperial colonies outside Europe. Traditional colonial powers like Britain and France sought to defend their colonies while new emerging imperial powers like Germany sought new colonies to expand their empire. Such factors resulted in increasing animosity between the different European powers which culminated in the war.