I'll just say it again. You haven't thought this through far enough.
I do not see how a world government would be any better at dealing with terrorist threats than national governments are. On the contrary, if a world government resembles the emerging E.U. government, it will be less capable. To get a sense of how bogged down the E.U. is already, consider that the proposed E.U. constitution is 500 pages long! That's hopeless.
- Jed
