The whole "unified earth" idea is viable, as long as the rule of law is viable. Unfortunately, because so many people are so hideously corrupt, it just ain't gonna happen. Taking a collection city-states and forging a nation was a difficult task, but was achieved because of a common ethnic/geographic identity--separate from others--and provided for a common defense. It was actually in the best interests of city-states to join together in order to fend off the wolves (take the principalities of Italy mentioned in "the Prince" standing side by side to fight off the French king, despite mutual hatred and jealousy).
We just don't have that kinda thing going on with this world. The EU does, against the US, and can create an "us vs. them" mentality which goes a long way towards creating political solidarity, of which Chirac is a poignant example. Muslim radicals are doing the same, originally with OPEC, and now with the anti-western philology. But remember how this turned out at the turn of the last century. Alliance building is not a good idea, no matter how powerful you are. America is not an isolationist as in the past century, able to march in as a second wind after everyone else had tired out. Things are going to be much different this time, and I assure you that there will be strange alliances formed. NKorea, Iran, Syria, Saudi Arabia and many other nations would have no problem seeing the US destroyed, despite any invested economic interests. There will always be someone else to buy oil...
There will also always be an enemy.
So long as a man stands proudly alone, there will always be several others banded together to pull him down.