First, there is a long tradition in the literature of international law that, although national sovereignty is an important consideration in world affairs, it is not sacrosanct. If a government treats its own people with terrible brutality, massacring them and such like, there is a right of humanitarian intervention by outside powers. The introduction of the offence of crimes against humanity at the Nuremberg Trial after the Second World War implied a similar constraint on the sovereign authority of states. There are limits upon them. They cannot just brutalize their own nationals with impunity, violate their fundamental human rights.
Is there then, today, a right of humanitarian intervention under international law? The question is disputed. Some authorities argue that the UN Charter rules it out absolutely. War is only permissible in self-defence. However, others see a contradiction between this reading of the Charter and the Charter's underwriting of binding human rights norms. Partly because the matter is disputed, I will not here base myself on a legal right of humanitarian intervention. I will simply say that, irrespective of the state of international law, in extreme enough circumstances there is a moral right of humanitarian intervention. This is why what the Vietnamese did in Cambodia to remove Pol Pot should have been supported at the time, the state of international law notwithstanding, and ditto for the removal of Idi Amin by the Tanzanians. Likewise, with regard to Saddam Hussein's regime in Iraq: it was a case crying out for support for an intervention to bring the regime finally to an end.
Just think for a moment about the argument that this recent war was illegal. That something is illegal does not itself carry moral weight unless legality as such carries moral weight, and legality carries moral weight only conditionally. It depends on the particular law in question, on the system of law of which it is a part, and on the kind of social and ethical order it upholds. An international law - and an international system - according to which a government is free to go on raping, murdering and torturing its own nationals to the tune of tens upon tens, upon more tens, of thousands of deaths without anything being done to stop it, so much the worse for this as law. It is law that needs to be criticized, opposed, and changed. It needs to be moved forward - which happens in this domain by precedent and custom as well as by transnational treaty and convention. I am fully aware in saying this that the present US administration has made itself an obstacle in various ways to the development of a more robust and comprehensive framework of international law. But the thing cuts both ways. The war to depose Saddam Hussein and his criminal regime was not of a piece with that. It didn't have to be opposed by all the forces that did in fact oppose it. It could, on the contrary, have been supported - by France and Germany and Russia and the UN; and by a mass democratic movement of global civil society. Just think about that. Just think about the kind of precedent it would have set for other genocidal, or even just lavishly murderous, dictatorships - instead of all those processions of shame across the world's cities, and whose success would have meant the continued abandonment of the Iraqi people.