Answer:
On one hand nationalism can be a unifying force that allows the people of one country to band as one against one common enemy. On the other hand, nationalism breeds an "us versus them" mentality and as such requires scapegoating and may lead to breaking or degrading ties with other countries. These scapegoats are usually the minorities of whatever the leaders of the nationalistic movement deem, whether it be the Jews in Nazi Germany, or immigrants coming into Europe.