Honestly it has to be a massive disaster, maybe aliens, but most likly a zombie apocalypse. Although normally I would assume by the end of this millenium there will be close to a world government, most nations of the world are already under some form of a regional agreement, aka nafta, eu, au, the organization for asia that i cant remember the name of, and china/japan/south koreas new free trade agreements. I think through economics we will see the possibly of world peace, what do you think?