In my opinion nationalism is only a positive force among people who share the same nationalistic view. When every nation is looking out for it's own national interest this creates competitiveness to be on top. Conflicting nationalities have caused the great wars we read about today, slavery, deaths, and international problems. Nationalism does provide more strength and power but often at the cost of other nations. Therefor it has had few benefits internationally. It has lead us to rely on military forces to solve problems and made us look away from the interests of our neighboring nations.
A good example of this would be after World War One. France and Britain had both signed treaties leaving them in control of much of the middle eastern territories and peoples. France and Britain had previously agreed to divide up these territories according to ethnic, cultural and religious nationalities. To thank the Arabic people for helping them fight the Turks and Germans they promised them their own independent home land. But their loyalty to the contract was not delivered. Oil had recently become valuable and the allies were concerned about securing a source. So instead the Arabic people were scattered throughout different nations and forced to share their nation state with several different cultural groups. The Arabian people were insulted and betrayed. But the allies were far too concerned about