To continue with what I was talking about earlier, and to address Dean's objections in the comments section:
Sovereignty does exist. That isn't in question. What's in question, then, is how one sovereign body can influence another. There are, of course, ways of influencing governments to do what you wish. First, of course, is the threat of war, occupation, or outright slaughter; it isn't "just", but it can exist, and can work quite well. The problem, of course, is that using that style of bargaining ("war is diplomacy by other means") tends to lead to a lot of dead people and destroyed property on both sides and is usually best avoided. it can also lead to unexpected side effects, and the possibility exists that the aggressor can lose, in which case it becomes vulnerable itself.
The other, and often superior way, is to appeal to the government's own self-interests. This is where the concepts of international law, international treaties, international bodies and bilateral and multilateral agreements come into play. None of these are sovereign in-and-of themselves- they simply represent agreements between sovereign governments to play by the rules if they know that the other guys will too. This doesn't remove the government's sovereignty, it simply provides a guideline by which that government can best use its sovereignty to avoid prisoner's dilemmas, tragedies of the commons, arms races, and all the other problems that can affect or disrupt security and trade. This even extends to the UN... if a state is not a member of the UN and not a signatory to its treaties, then the UN has no authority over it, and it need not pay attention to a single word the UN says. This also includes less formalized agreements- the evolution of the current international system may have got its start at the Treaty of Westphalia, but it's largely informal. Nothing wrong with that- tradition can be even more powerful than law.
The problem, of course, comes up when somebody makes a deal and then breaks it. On a certain level a sovereign nation (such as the United States) is perfectly free to attack whomever it wishes, with the understanding that it is in return a target that can be attacked itself. However, most sovereign nations (including the United States) is beholden to a number of treaties and bodies that have set down ground rules by which it can operate, under the expectation that it can enjoy the knowledge that everybody else who is a signatory to the treaties (or whatever) is playing by the same rules. If somebody signs a treaty and then breaks it (or agrees to be involved in an international body and then ignores it), then the whole thing falls apart, because the expectation that everybody else has that you were going to play by the rules disappears, and they immediately ignore the rules as well; nobody likes a free rider, and nobody likes being a sucker.
(Yep, we're back to game theory again... free riders are those who enjoy the benefits of a collective action without having to be subject to the rules, and suckers are those who are subject to the rules but don't recieve the benefits. The one creates the other). In essence, this is why so many people get so (legitimately) pissed off at certain states (including, notoriously, the United States)- signing and breaking a deal pretty much ruins it for everybody, including (usually) the party that is breaking the deal.
So how does this enter into Iraq? Well, Iraq is part of the U.N., and is subject to the unwritten traditions that underlie the international system. The United States is part of the U.N. too. When Iraq breaks a deal (as it did when it invaded Kuwait) it opens itself up to retaliation: from Kuwait itself, its allies, and the members of the international bodies and treaties that it has agreed to be subject to (either explicitly or tacitly). The severity and nature of the deal-breaking action usually determines the reaction, of course- nobody is going to suggest invading Hong Kong, for example, because it plays merry hell with international copyright laws. They might sanction it, or cut off trade status, but they aren't going to invade. In the case of the Gulf War, though, Iraq had already invaded one country and was threatening others, the U.N. was expected to respond, and therefore authorized the action to liberate Kuwait from Iraq- both because Iraq had broken its deal, and not freeing Kuwait would break the deal the U.N. and other countries had with it.
(Incidently, this is also why the Gulf War wasn't "just about oil"- oil may have been the reason why it was prosecuted so quickly, but the U.N. and U.S. had perfect justification to do what they did.)
A similar justification existed for the invasion of Afghanistan- the government of Afghanistan (or at least an allied body) had directly attacked the United States and it threatened to do so again. The United States was perfectly justified in what it did- even if the U.N. hadn't approved it yet, there's no doubt that the U.N. (at the very least the Security Council) would have. (The taliban was not a member of the U.N., however, so it isn't quite so clear as it was in the case of Iraq.) It's important to note, however, that the attack had already taken place- anticipatory attacks are by-and-large not kosher- we'd end up with thousands of wars between governments that are hostile to one another and are absolutely convinced that the other guy is going to attack first and they're simply acting "in self-defense". (Or they'd simply use that as an excuse. Not that it would really matter, in the long run.)
Now, of course, it's a different story, because while Iraq is still breaking deals by not letting the inspectors in and (presumably) creating weapons of mass destruction, only one nation (the United States) is saying that this is grounds for invasion. More importantly, though, that nation isn't satisfied with simply ensuring that Iraq plays by the rules.. it wants to force regime change in order to further its own interests and security. (Which no state agreed to, and no state is likely to do so). If the United States invades, then all of a sudden its the deal-breaker- it's the free rider that is attempting to enjoy the benefits of international treaties and bodies without having to be constrained by their rules. Whether this is "just" or not is immaterial- it's a simple matter of contracts made and broken, and the United States will be breaking practically every contract it has made by doing so, and in so doing make those contracts meaningless. Without those contracts, of course, all you have is anarchy.
That is why the rest of the world is pissed. That is why the warbloggers who have been calling for invasion are apparently either completely unaware of how international law and international relations work, or have ceased to care because of the United States' power. That is why Europe is going absolutely batshit right now, because they place great value on these sorts of treaties and bodies. And that is why any invasion of Iraq is something that needs to be considered very carefully, and probably is the reason why George H.W. Bush didn't take out Saddam in the first place. Yes, the United States could invade Iraq. It's sovereign, it has that ability, and could probably go it alone if necessary. By doing, so, though, it risks tearing apart the foundations of the vast majority of the collective and bilateral security agreements and bodies that keep what is an incredibly anarchic system somewhat peaceable and orderly. It becomes a free rider, and the rest of the world will know that, and will know full well the implications of a hyperpower breaking practically every deal it has made.
(There is, of course, another way of creating order in an anarchic system. It's one that has become increasingly popular lately, and serious people are starting to advocate it, despite the problems that it has had in the past. It's a system where one government gains sovereignty over most of the rest, and ensures security through its own strength. You probably know that system. It's called Imperialism.)