By no means am I an expert on international law, so perhaps my opinions on whether the war in Iraq is legal are not are pure bull puckey. At any rate, here is my understanding of the situation:
In 1991 Congress declared war on Iraq, and shortly afterward Iraq was militarily defeated and a cease-fire was negotiated.
During the period 1991-2003 something called a peace treaty which would end the war was never approved by Congress (or even agreed to by the belligerents) and thus not made into law, whether international or domestic. So the war with Iraq was never formally ended, merely concluded with a cease-fire. Thus, the US has been at war with Iraq for the period beginning in 1991 up to the time of invasion in 2003 and possibly even the present. Notably, Iraq frequently violated the terms of the cease-fire by firing missiles at USAF jets patrolling no-fly zones in N and S Iraq which the US had the right to patrol under the terms of the cease-fire. By my understanding, violations of cease-fire agreements justify resumption of the hostilities by both parties, and in this case the US was the victim of a resumption of hostilities, not Iraq.
In 2002 (possibly prior), the Bush Administration decided to topple Saddam Hussein's regime, citing WMDs and terrorism as the justification since a few missile launches that did not significantly damage US military operations in the no-fly zones makes for a harder case for resuming the hostilities to the public. Although Saddam did not have WMDs, he was materially supporting terrorism (for instance: giving $15k to the families of each Palestinian suicide bomber). Since Israel is a US ally, providing aid to parties who perpetrate acts of war on Israel is an act of war against the US as well. Any proof of Saddam providing support to terrorists intending to directly strike the US or its interests also shows that Saddam engaged in acts of war against the US itself in violation of the cease-fire agreement.
Now, the morality and justifications of this war are subjects of hot debate. But the technical legalities of it shouldn't be, according to my understanding of them. President Clinton had the same legal right to do what Bush has done based on Iraq's repeated and persistent violations of the cease-fire terms. Prior to the renewal of hostilities in 2003, Bush made the case to the UN in a way that makes the public (notorious for forgetting what the Executive branch did 12 years previous) in the US think that he broke the law.
Bottom line: IMHO, the war is legal. It may or may not be justifiable by other criteria, but there is a difference between what is legal (meaning permissible under the law) and what is moral/ethical/rational and people should know the difference when they debate the war and whether it is justified.
Now let me reiterate: I am not a lawyer, and I certainly don't practice law in the international arena. I could be dead wrong in my understanding of the situation.