Merriam-Webster defines Nationalism as
1: loyalty and devotion to a nation
especially: a sense of national consciousness (see CONSCIOUSNESS sense 1c) exalting one nation above all others and placing primary emphasis on promotion of its culture and interests as opposed to those of other nations or supranational groups
Intense nationalism was one of the causes of the war.
For a modern day example, one should look no further than Germany’s Adolph Hitler and the atrocities he commited during his reign in the name of Nationalism against Jews.
Speaking to a UK audience, conservative commentator Candance Owens believes his Nationalism wouldn’t have been so bad had limited his activities to Germany but where he went wrong was wanting to practice Globalism.
In other words, the atrocities against Jews wouldn’t have mattered had Hitler not had aspirations of wanting everyone in the world to be German and kept his practices within the German borders.
For reactions to Owen’s assertion, flip the pages.