In 1991 a conditional ceasefire was signed between the coalition forces and Iraq, over a period of ten years Iraq made no effort to comply with any of the conditions of the cease fire and went out of their way to obstruct anyone tasked with ensuring compliance. Hostilities resumed in 2003. The war was not illegal, no further UN mandate was required. The UN were approached as a courtesy but without the knowledge that several countries that were members of the Security Council had their snouts firmly in the corruption trough by that time.
Had the war been illegal half the lawyers in the USA and many from the UK and elsewhere would have been queueing up to file their lawsuits long before now.