Several things have come to light since then, not least Tony Blair's statement that he would have found another pretext for invasion if not WMD. Jonathan Steele says:
Apart from WMD there was no other conceivable [legal] foundation for an invasion. Using force to produce regime change on humanitarian grounds is not permissible under international law, and the attorney general told Blair as much in July 2002. There's a chorus to condemn Blair now. It remains the case, however, that having no foundation in law to remove a regime on humanitarian grounds is not a satisfactory state of affairs. Governments that commit appalling large scale crimes ought to be vulnerable to removal by force approved under international law...at or very close to the time that those crimes are committed. 
 P.S. see also Hans Blix and Ken Macdonald
 In his Nobel acceptance speech, Barack Obama observed that the conditions that led doctrines such as a 'responsibility to protect' in the 1990s and early 2000s have not gone away:
wars between nations have increasingly given way to wars within nations. The resurgence of ethnic or sectarian conflicts; the growth of secessionist movements, insurgencies, and failed states -- all these things have increasingly trapped civilians in unending chaos. In today's wars, many more civilians are killed than soldiers; the seeds of future conflict are sown, economies are wrecked, civil societies torn asunder, refugees amassed, children scarred.The prospects for co-ordinated international humanitarian action have, however, deteriorated.