Due to the extreme nationalism in countries like Germany for example, Nazism was able to become a unification movement in Germany where people had so much pride for Germany and its ruler at the time, Hitler, they fought on the side of Hitler and what he believed in. The Nazis killed millions at an attempt to ethnically cleanse the country from various minority groups.
Nationalism can have many different negative effects, especially concerning history and nationality. One of the most common examples is during the Holocaust, which caused one of the most detrimental wars in history, World War II.
The extreme nationalism in Germany was one of the causes of World War II, a war that resulted in millions of causalities in countries worldwide.