The essence of government is territorial monopolization of law and security. Global government, then, would be a worldwide monopoly on law and security. Naturally, the monopolists of law and security that exist today (national governments) will be reluctant to give up their territorial monopolies to a global monopolist. However, faced with the prospects of losing their monopolies (by increasing competitive production of law and security by private individuals within their respective jurisdictions) or joining a larger cartel and preserving the status quo of non-competition within their jurisdiction (at the cost of yielding ultimate control to a higher, global monopoly of law and security), I think national governments will rationally choose to cartelize.
However, cartels are inherently unstable. In this case, each government stands to profit at the expense of other governments by providing a "better deal" in terms of lower taxes and better infrastructure over against its "competitors", that is, other governments. Citizens are attracted to countries with large welfare entitlements and profit opportunities (for some time, America has managed to offer the seemingly impossible: both).
Successful cartels (OPEC, Federal Reserve, etc.) are ultimately bound together through threat of retaliation/force against any member who tries to cheat on the cartel. If you were determined to build a global government, the key obstacle is how to make a meaningful cartel agreement between independent, totally sovereign nations. In the United States, we have the Federal government to enforce the cartel between the state governments. It has, in effect, become a monopolist of territorial jurisdiction, leaving very little to the independent self-direction of the states themselves.
Here are some steps that would likely need to happen to begin cartelizing national governments in preparation for a global monopoly on law and security:
1) You would want to build a worldwide financial cartel. This is accomplished by creating central banks (currency monopoly) in every country. These central banks, by operating in unison, can act as a financial cartel. Note that each central bank has an interest in defying the cartel, that is, to inflate less than the cartel (thereby becoming a "hard" currency). The hard part is forcing the central banks to operate in unison. One solution to this problem is consolidation (e.g. the European Central Bank). Once you have a worldwide central bank with the power to inflate, the rest is just details because this bank has the power to print as much money as it needs to buy all the other pieces to build the global government.
2) You would need to build a worldwide legal cartel. This is both the most important and the most difficult part. However, with enough money, it can be made to happen. You just keep buying off the local sovereigns until they put the agreements in place that make your World Court the de facto court of final appeal.
3) Worldwide security cartel. This is the lynchpin - without this, you cannot put teeth into (1) and (2), but without (2), you cannot have (3), which is why (2) is the most important piece of the puzzle. The security cartel is the overwhelming force by which the global government can overpower the dissent of any particular local government. The United States is the perfect example. No individual state would ever try to stand up to the Federal government because the Federal government has overwhelming force because it collects taxes from 50x the population (and has the central bank with which to inflate, as well) that any individual state has to collect from.
What specifically got me thinking about writing this post was point (3). I was thinking about the splinter nature of special operations units. In most countries, special ops units are not part of the regular chain of command, in fact, bypassing it and going straight to the top. The special ops from many different countries frequently train with one another, so there is a lot of "cross-pollination" and handshaking that goes on between these units. Now, imagine a global organization that links all the special operations units in the world together for the purposes of "combatting global terrorism", for example, and imagine that a particular nation tried to resist the globalization agenda. Now, even if it was a very large and powerful nation, such as the US or Russia or China, if that nation's actions could be characterized as "terrorism" or a general threat to worldwide welfare, all the special ops splinter units could be assembled into a single body and sent up against that nation. A very powerful nation's regular units would not likely fare well against special ops units which are many times more hardened, experienced, determined and flat-out deadly.
Since this is a rational conclusion, it would likely never come to this in any case. The belligerent nation, once confronted with the possibility of an alignment of these very effective special ops units from all over the globe against itself, would likely see the light and become cooperative.
Just a thought I had.