How is nationalism negative what are examples?

Before nationalism a Serbian prince would feel a deeper connection with his horse then with the farmers that work for him (Ernest Gellner, 1998). After Nationalism you could imagine the opposite, it unites the people within a nation together and they gain a stronger relationship to stick together and to keep their national identity intact. One of Gellners's theories or opinions if you will, that Nationalism is only a product of Imperialism to create a stronger economy in a particular nation. It can also cause several nations to have a very strong relationship due to common values.