How did nationalism help cause World War I?
It's one of the factors in causing WW1.
Germany practiced nationalism by improving its military ( militarism ) and Germany wanted to be the top 1 country in Europe. Other nations grew tense due to the fast improving military of Germany.
Russia believed in Pan-Slavism. Pan-Slavism is the belief that the Russian race is superior compared to other races. So Russia colonized smaller countries to spread the Russian culture. Other than Pan-Slavism, there was also Russification.
Russification is a culture assimilation where non-Russians give up their language and culture for the Russian culture and language.