Yes, nationalism can be a force for good, because it can unify a group of people. A nation can provide unified self-defense, and a national pride. A national government can create uniform money and national laws to allow for ease in commercial transactions and trade of good. If nationalism leads to stability, national identity, and pride, it is a good thing.
No, nationalism can bring benefits, because it can unite people. After September 11, Americans held a large fundraiser. The same thing happened after hurricane Katrina. Because people are proud to be American, they feel solidarity with other Americans. That makes them eager to help each other. American nationalism makes people supportive of each other and kind.