Ultra-nationalism is an overtly strong identification to a society.
"Often, it is the belief that an ethnic group has a right to statehood, or that citizenship in a state should be limited to one ethnic group. It can also include the belief that the state is of primary importance, or the belief that one state is naturally superior to all other states."
Ultra-nationalism is sometimes a response to an event. During World War II when the US fought the Japanese we rounded up US citizens, of Japanese desent, and placed them in internment camps. This can be seen as a form of ultra-nationalism.
Ultra-nationalism is important becasue it can shapes society and governments. It may lead to war, genocide and other atrocities.