Matthew Yglesias and Robert Musil (the Man Without Qualities) have been having a discussion about why the international community should recognize illegitimate governments, which was kicked off by this post here. As it's short, I'll post the vast majority of it, and comment as I go:

It also seems an appropriate day to consider some principles which events of this day not really so many years ago helped to establish. One very odd aspect of international law is its failure to incorporate some basic eighteenth century political insights. For example, political thinkers of the eighteenth century (including the signers of the Declaration) recognized the universal principle that the only source of governmental legitimacy is the consent of the governed. And "consent of the governed" means frequent, free and fair elections - not some dictator telling the world that if an election were held he would win. And "consent of the governed" doesn't mean some dictator or oligarchy explaining that their country is not "ready" for full electoral democracy. These are universal democratic principles, not details subject to ad hoc consideration on the basis of cultural relativism. There is room, however, for various uneasy "virtual representation" theories which disenfranchise some classes of people (children, criminals and others), and for significant constitutional variations.

Bolding mine, and for a good reason: although this is consistent with American conceptions of legitimacy, it does not and never has meant a hill of beans historically. Robert, put down the Federalist Papers and go read some Hobbes- legitimacy does not come from elections, it comes from the consent of the governed (the subjects) to obey the will of the government. If that consent is withdrawn, then (as John Locke pointed out) the alternative is armed revolution or (as Thomas Hobbes pointed out) leaving that country. Free and fair elections mean that it's more likely that the government is representative and reduces the likelihood of the necessity of armed regime change, but it doesn't eliminate it... that's the entire reason the second Amendment exists, and it's a pretty important concept in international relations. Indeed, free and fair elections don't necessarily have anything to do with important elements of modern states, or else how would the Queen of England gain her ceremonial (and very real, if only symbolic) power as the head of state of not just England, but a fair chunk of the commonwealth as well?

Hell, Rousseau and other critics of liberal democracy (including most Anarchists) would argue (and did argue) that "free and fair elections" are meaningless if certainly things aren't "up for grabs", a concept that formed the basis for Lockean liberal democracy. Rousseau famously said that "people in democracies are sovereign only on election day", and it's a sentiment that exists to this day.

And how can something be a "universal principle" when most of the world doesn't recognize it, recognizes the legitimacy of non-democratic governments and is willing to normalize relations with them? The United States is (notoriously) no exception to this rule. You can't arbitrarily declare that a principle is universal.. it has to be empirically demonstrable, and by no means is the Declaration any kind of empirical demonstration of the universality of the principles it sets out. It is the justification for rebellion by the Founders, yes, and important in that respect, but it doesn't mean a hill of beans outside of that nation's borders. They may hold these truths to be self-evident, but I doubt the British agreed at the time, and I doubt a lot of the planet would even now.

But most forms of international law continue to inhabit some pre-Enlightenment sphere in which even a simple dictatorship can claim to be the legitimate government of a nation with rights to defend itself and the nation. This form of international law dominates the United Nations. And it is completely anacronistic and wrong.

If by "anachronistic" you mean "traditionally the case", then yes, the concept of national sovereignty has been observed for (I believe) longer than the United States has been a country, and certainly longer than any of us have been alive. It recognizes what I just said above: that legitimacy does come from the people, but not from elections: it comes from their willingness to be subject to its authority. (This is true in the United States as well- the difference is that the final authority is the Constitution, not Congress. In other countries it's entirely different- in Britain, for example, it's the Queen and her Parliament that is supreme, and the constitution that is subject to the will of that Parliament.) If the people didn't obey the government, if they didn't agree to be subjects, then that government has no legitimacy. Period. End of story. Even if it slaughtered them all like hogs, it still wouldn't gain legitimacy, as what's a government without the governed, and how can you govern the dead?

(Besides, dictators can be popular and even beloved by their subjects. Unlikely, yes, but certainly possible. What if the general will was that they wanted the El Presidente to be dictator-for-life, and were willing to defend him against those who attempt to impose a democratic system? Is he not legitimate then?)

One consequence of these universal principles - a consequence which is not openly vetted in what E.B. White called the little green glass shee-bang on the East River - is that all governments that do not conform to such principles are illegitimate.

Again, the concept of "universal principles" is deeply problematic within the political sphere- precious little is universal, everything is up for debate, and assertions that any system is The One Best System are usually quickly and brutally shot down by those who point out the flaws of that system. (As I said earlier, this includes liberal democracy.)

In other words, governments that do not conform to these "universal" principles don't lose their legitimacy- their subject-derived legitimacy invalidates the universality of the principles. A distasteful notion, perhaps, but just try getting a few beers into an Anarchist and start hearing about how screwed up liberal democracy is. That's why national sovereignty is, in the end, so important: if countries tried to impose their vision of good government on everybody else, the world would never cease warring, and most people would, unfortunately, be too dead to argue.