Yonatan Lupu makes some good points about Kofi Annan's recent declaration of the Iraq War as "illegal":

To begin with, the question of whether the Iraq invasion was illegal under international law is far from settled. By expressing his opinion with such certainty--not to mention with timing that seemed explicitly political--Annan undermined the complexity and seriousness of the international legal regime he claims to be defending. Annan's argument is rather simple: that the invasion was illegal because the U.N. Security Council did not pass a resolution in March 2003 explicitly authorizing it. This assumes Security Council authorization is required for the use of force to be legal; but in 1999, when nato forces attacked Serbia without such approval, Annan did not call it "illegal."

Critics of the war are loathe to discuss this subject...I wonder why?

The author goes on to point out that what Annan mostly pointed out with his statement was the unwillingness and inability of the U.N. to do anything about it:

Moreover by asserting the invasion's illegality 18 months after the fact, Annan has, in a sense, highlighted the irrelevance of that illegality. The Security Council may not have explicitly authorized the invasion; but neither did it prevent the invasion from happening. And even if Annan is right, those responsible for the invasion--Bush, Blair, Rumsfeld--will clearly never face any sort of trial or sanction. What, then, are the practical consequences of the Iraq war's alleged illegality? Annan's comments last week were in part a resentful admission that there are none.

To recap: By selectively ignoring the illegality of some non-UN-endorsed military actions, while condemning others, the UN has set a double-standard. If anything, the invasion of Iraq had much more international endorsement and legal standing than NATO's intervention in the Balkans.