Nationalism is a belief in the supremacy of the nation, its economic, political, cultural and religious structure. It is an intense form of patriotism and causes people to place their nation before anything else. Nationalism in Europe was a major cause of World War I.
The nationalism developed across Europe, both in major powers (such as Britain, France and Germany) and their colonies or annexed states (such as parts of the Russian Empire or Slavs and Bosnia in the Austro-Hungarian Empire). For the citizens of powerful nations as such Britain and Germany, a firm belief in their military prowess and colonial power spread and led to nationalism. Similarly, in annexed states such as Serbia in Austria-Hungary, people united against the colonial powers to preserve their culture and way of life. The literature of that era, including stories, poems, and newspapers, supported nationalism. For example, "The sun never sets in British Empire" was a common saying in Britain. The rise of nationalism also generated distrust of other nations due to cultural differences and political ambitions.
All this led to the development of powerful nations and powerful groups within a nation and caused World War I. In fact, the world war started out as a result of the murder of Archduke Ferdinand of Austria-Hungary by pan-Slavic nationalists.