Why do you think nationalism is bad?
Patriotism is love for one’s country. Nationalism is the belief that one’s country (and often ethnic subset) is supreme (and more deserving of prosperity), speaking generally.
Trump’s campaign rhetoric is emblematic of nationalist themes. Anti immigrant. Playing up cultural mythos. Bragging about military capabilities. Blaming foreign nations for most problems. Implying that a return to core values will bring greatness. Lots of fear and anger.
Tends to be very effective for groups of people that used to be powerful but are no longer.
If that's too much recursion, consider the World War I and II (among many others), both being based essentially in nationalism.