Question

how did nationalism cause world war 1

Answer 1

The assassin of Archduke Ferdinand which triggered the war was a member of a nationalist group that fought for Serbian independence. They believed the Archduke's assassination would help them achieve their aims.

Answer 2

The answer above is true, but nationalism is being really devoted to your country, Nationalism caused competition between the great powers, like England, Germany, France, and Russia which lead to secret alliances. It also lead to ethnic groups wanting there own country. The war was so huge, because of secret alliances joining to protect there allies, and since almost every country belonged to the british empire.