There are some people, quite in the minority in my opinion, who sincerely favor the idea of a global government, often under the rationale that the nation states need to be kept in check. The problem with this view, however, is exactly the same problem of nation states. The proponents of global government often correctly point out that nation states have no arbiter above them to keep them from overstepping their jurisdictions, and that this therefore leads to conflicts between nation states.
But a global government would also have no arbiter above itself to keep it in check. In fact, if we go through the levels of government in America from bottom to top, the city lacks an arbiter above itself so we have states, and the state lacks an arbiter above itself so we have a federal government. Any government period lacks an arbiter above itself, and adding another level of government on top of it doesn't solve the problem because it too now has no arbiter above itself. Note that nation states came about as a result of the exact same complaint (the lack of a higher arbiter) that is being made by global governmentalists of nation states.
If the problem with nation states is that they become too powerful, then creating an even more powerful government with an even larger jurisdiction, over the entire world, does not solve this problem. It inflates the problem to the global level. How would we be able to keep our global leviathan in check? The proponents of global government present no logical answer for this.
A global government, by its very nature, would have to have a larger jurisdiction than a nation state. Further, it would have to have a larger budget, which inherently means that it must tax, spend and inflate more than a nation state. It also implies one global currency determined by global central banking. Global inflation created by global fiat currency (which already exists to a certain extent due to global hegemony between nation states) would wreak unimagined havoc on the world's economy. If the complaint of our global governmentalists about nation states is that they violate the sovereignty of other nation states, which is true, this complaint rings hollow when one realizes that a global government inherently must also violate the sovereignty of all nation states.
What effect could adding an even higher level of government be but to centralize power even more than it is within a nation state? Further, how is it possible to centralize this power without inevitably favoring different nation states over others, hence creating a global government biased either towards whatever region has the highest population or towards whatever areas are favored by the administrators of this global state? The idea that a global government can be 100% neutral to and/or equally representative of the nation states (global democracy) is an absurdity when applied to real life. The "top headquarters" have to be put somewhere, and it is thus inevitable that a global government would, to some extent, be an ivory tower that represents the area that it resides in more than areas outside of its immediate periphery.
If the proponent of global government wishes to sneak their way around these problems, they will propose the idea of a global government that effectively abolishes the nation states; that a global government scenario would not be like a federal global government representative of and/or attending to nation states, but that it would replace the nation states entirely. The only vision I can get from this idea is a big "blob".
But this scenario is even worse than the previous one, for now we simply are left with a single, highly centralized entity that controls the entire world. It leaves no room for any quasi-sovereighnty of nations, states, counties or cities. Such a scenario, I believe, is impossible to truly put into practise because the very nature of the wide distances between geographical locations. It is impossible to keep the entire world under the rule of one single governmental authority. The force of individual diversity and the mere distance between geographical locations would make it impossible to create such a static, "stable" atmosphere.
I would propose that the true answer to the problems of nation states lies in the opposite direction: decentralization of power. The reason why nation states start violating the sovereignty of eachother is because they are easily prone to become overly centralized, and thus in possession of exessive powers, which manifest themselves in jurisdictional grabs and expansions. It is the empirial impulse. The global governmentalists merely propose to replace this with an even larger empire that is prone to the exact same errors.
When nation states are decentralized into more compartmentalized states, such jurisdictional abuse becomes less and less possible. And when those compartmentalized states are decentralized into more compartmentalized cities, such jurisdictional abuse becomes less and less possible. A city doesn't possess the means to wage a war half-way across the world in conquest of other peoples. No, the problem of nation states become more solvable when we persue localized ways of solving problems, hence aleviating any real incentive for larger entities to act.
When nation-states are restrained by law to only have powers over a limited number of things and to confine themselves to their jurisdictional territory, they function in a much better fashion. When nation states have laws restraining them from violating the jurisdiction of lesser entities, such as parishes and towns, they function in a much better fashion. However, it must be said that it is possible for nation states to overstep their bounds despite such laws because of the nature of government in itself. Therefore the lesser governmental entities have a responsibility to actively keep the higher entities in check. Thus, the solution to the problems of nation states does not lie in global government, but in the application of sovereignty to considerably more local government.
In terms of logical consistancy, one must apply decentralization down to the individual level. If a nation state is a sovereign entity that should not be breached by a global state, then a state such as Texas is a sovereign entity that should not be breached by a nation state, and the city of Galveston Texas is a sovereign entity that should not be breached by the state of Texas, and the individual person in Galveston is a sovereign entity that should not be breached by the city, state or nation state. The end result is that of the sovereignty of the individual, which should not be breached by any "higher" entity. The consistant application of this would be true individualism.
I would also suggest that the global governmentalists must apply logical consistancy in their direction as well. If a nation state must be kept in check by a global state, then a global state must be kept in check by a galactic state, and a galactic state must be kept in check by a cluster state, and a cluster state must be kept in check by a universal state. Essentially, the global governmentalists must accept the idea of a universal government. Into the sky we go (head explodes)! It should be noted that when we follow this route there is no resolution (what is to check our universal state?), it is a paradox that keeps reasserting itself, while when we go the route of decentralization it stops at the individual. The person who takes the notions of galactic or universal government seriously has read one too many science fiction novels and took Star Wars way to seriously.
Global governmentalists should reconsider their position and perhaps adopt one more favorable towards the idea of decentralizing the nation states, as opposed to controling or replacing them through global government. The voluntary and non-violent decisions of individuals and the communities that they constitute should not be co-opted by nation states or global states. Those who favor global statism are proposing to solve the problem of national statism by enlarging it to the entire world, and among them are the staunchest of neoconservatives who strongly favor the utopia of "global democracy". It is thus a nonsensical, self-contradicting ideology, as are many anti-freedom ideologies.