(1) My own view on whether there is a basis in international law for humanitarian intervention in situations of this kind is that there is. As I have already stated this view at some length, I will be brief on the present occasion. There is not only a right, there is a duty, of humanitarian intervention when a government is committing mass atrocities against a civilian population. This can be established by reference both to customary international law and to the doctrine of A Responsibility to Protect, underwritten by the UN. The question, in particular, of whether a UN resolution mandating intervention is required can be quickly answered - no - for a reason given here: 'The U.N. Security Council is not the sole or unique custodian about what is legal and what is legitimate'. To put the same thing another way: a system of law that would countenance mass atrocity without any remedy simply because the interests of a veto-wielding power at the UN blocks remedial action is morally unacceptable, indeed intolerable; and so where the UN itself becomes delinquent by not upholding some of its own most fundamental principles, the UN not only may, it should, be defied by member states willing to give those principles more respect.
(2) However, integral to the doctrines of humanitarian intervention and R2P alike is the requirement that a prospective military intervention should have a reasonable chance of success. Intervention is not to be contemplated without regard to the likely consequences. In the present case, this is, in my view, the most difficult of the three issues to resolve. Would military intervention against Syria now do any good? That depends, of course, on what its objectives are: whether to influence the overall outcome of the civil war in that country; or merely to weaken the regime's military capabilities; or to deter it from further gas attacks on the Syrian people; etc. I don't propose to offer answers on each different conception of possible objectives. Indeed I don't know that I can. My earlier uncertainties over Syria have not dissipated. But, in any case, one should note that intervention may be justified even if the overall balance of consequences is not beneficial.
(3) For intervention may be undertaken on retributive grounds, to punish a regime that so blatantly flouts the norms of international humanitarian law and the principles of all civilized morality. It may be regarded as morally unthinkable that such a regime should be able to commit gross crimes against humanity with impunity - without being made to suffer any significant penalty. In this situation military intervention is undertaken as a reprisal (scroll to the end) for the crimes committed.
How one weighs the force of (3) against that of (2) in a case where there may be negative consequences I am unsure. But it is these considerations rather than UN authorization or lack of it that should take precedence.