Nationalism doesn't need to equal "screw all you outsiders". It can be a wish to maintain the right of self-determination, a wish to maintain your traditions and ways in the face of a wave of global culture. You can be a "nationalist" in the sense of wanting to maintain these things, but still support greater global cooperation on important issues.
As for "the world under one banner" I doubt we'll see it anytime soon. Moreover, I hope we won't. The larger a nation becomes, the more complicated it becomes to manage well, and the more bureocracy you get, until the point where everything becomes so tied down in red tape that decisions are sent on endless hearings without any result. Any world government would have to work on the macro-scale only, dealing with issues that are global and leaving a lot of the decision-making to individual states, to ensure a somewhat fair chance for actual democracy.