Several pages ago, I asserted that good legal arguments could be made pro and con Syrian intervention; that legal arguments are therefore not critical to the question; but that the question should be decided on the basis of underlying policy(ies).

Ken Anderson has done the heavy lifting by outlining five different international jurisprudential viewpoints: Five Fundamental International Law Approaches to the Legality of a Syria Intervention (by Kenneth Anderson, September 5, 2013).

First his introduction and posited facts:

Lurking behind international law arguments over a possible US armed intervention in Syria without Security Council authorization are fundamental divides over the nature of international law itself. These divides include its authority with respect to sovereign states, the sources of authority it draws upon and methods of interpretation, and the degree to which it is essentially a “closed” system of law that looks to itself for answers and legitimacy or an “open” one that does not exhaust all the possibilities of legitimacy and action. These deep differences in conception and approach to international law are not simply intellectual arguments without practical implication; on the contrary, they account for much of the sense that the several sides in these international law debates somehow fail to address each other’s arguments.
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The factual assumptions are those made in the ASIL essay: first, the now largely undisputed claim that the Assad regime engaged in a major chemical weapons attack; and, second, the United States will undertake some military response even if it has no authorization from the Security Council and even if reasonably certain that such authorization, if requested, would be blocked by Russia and possibly others.
Here are the five legal philosophies that have been argued - each with a short snipped description (much more in Ken's article):

1. Formalism as Positive Charter Law. Formalism looks to the “formal” law for answers and, moreover, tends to treat those answers as dispositive – a “closed” system in the sense registered above. Any other concerns of policy have to be drawn into the formal system in order to count as law, and law as such trumps policy. Moreover, there is a hierarchy of sources, and the UN Charter is at the apex. Thus, the clearest formalist argument is that US armed action against Syria, irrespective of the reason in morality, politics, or policy, without authorization of the Security Council, violates the plain language of the Charter ....
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2. Formalism Beyond Positivism. Formalism is not the same as strict Charter positivism, however. Positivism looks to the “positive” law, particularly as written in the Charter’s provisions. But international law, even on a formalist view, includes customary international law, which is evidenced by state practice and the opinio juris of states indicating that their practices are driven by a belief that they act from a sense of binding legal obligation. This is part of formal international law, but important parts of it will always subject to interpretive debate. It is not beyond reason that even some “formalists” would be willing to go beyond strict Charter positivism and acknowledge that the Charter’s wrapping of customary international law of “inherent” self-defense into Article 51 can be seen to create a certain question as to whether the “inherent” rights of self-defense, that the Charter purports not to “impair,” is both formal international law but not necessarily qualified by the Charter’s language of “armed attack.” ...
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3. Responsibility to Protect (R2P). Neither formalist positivism nor formalism-beyond-positivism yields that sought by those seeking intervention on the basis of humanitarian concerns. They offer an argument that is essentially morality and policy – humanitarian necessity. The best formal legal argument is a descriptive claim offered as justification under the moral argument; mass atrocities are indisputably a gross violation of international law, and that command of law justifies a response under international law itself. It is expressed as a responsibility to protect, and it is – on this argument – both compelled as a legal conclusion arising from the underlying prohibition on mass atrocities and, moreover, something that has gained sufficient acceptance in the community of international law to qualify as, if not necessarily “law,” at least not contrary to international law. The question is where international law locates the “remedy” called for by R2P – cabined within the Security Council’s authority exclusively or not? ...
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4. “Illegal but legitimate.” One approach that has gained a certain traction is to frankly acknowledge that intervention is illegal under international law, but assert that international law is trumped in such a case by emergency and necessity, in this case humanitarian emergency and necessity. This might be understood as a peculiar form of the strong formalist position – which, recall, had two distinct conditions, first, that one looked to “formal” sources to determine the law and, second, that this formal law take normative precedence over everything else. This “illegal but legitimate” claim is one that, since first offered in the Kosovo intervention, has had peculiar attraction to otherwise formalist international lawyers; it allows them to continue to accept formalism’s “descriptive” condition while abandoning its second, normative claim that the formal law trumps. ...
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5. “Pragmatic” International Law. Perhaps out of implicit recognition of the methodological and conceptual difficulties in arguments for intervention based in R2P and humanitarian intervention, along with a political desire to confine the “war aims” of an intervention as well as its actual scope, the US government has made the issue not humanitarian intervention, emergency, or necessity, but instead the violation of what Secretary of State Kerry, President Obama, and other officials have described as the long-standing international norm against the use of chemical weapons. The central argument as it stands now is not that the chemical weapons sharply worsen the humanitarian situation – though of course that is true – but instead the defense of the norm against any use of chemical weapons. ...
Pragmatism (which is the rest of Ken's discussion) tends to be a two (or more) -edged sword; multiple good arguments can arise from different practical values being chosen (e.g., Putin vs Obama).

See, Michael Glennon, The Fog of Law: Pragmatism, Security, and International Law (2010); Fixing the UN : A fractured planet needs pragmatism (by Michael Glennon, April 23, 2003):

BOSTON— Some day, following the collapse of the international security system this winter, policymakers will return to the drawing board. When they do, one lesson is that rules must flow from the way states actually behave, not from the way they ought to behave.

"The first requirement of a sound body of law," wrote Oliver Wendell Holmes, "is that it should correspond with the actual feelings and demands of the community, whether right or wrong." This insight will be anathema to continuing believers in natural law, the armchair philosophers who "know" what principles must control states, whether states accept those principles or not. But these idealists might remind themselves that the international legal system is voluntarist. For better or worse, its rules are based upon state consent. ...
Ken Anderson, Living with the UN: American Responsibilities and International Order (2012); free download of the 1st 121 pages (SSRN link):

As a policy message for the US political system, it means two fundamental things: American conservatives need to understand that the UN is not going anywhere; it is a permanent feature of the international landscape, and they have to elaborate policy heuristics to deal with that permanence. American liberals, by contrast, need to understand that the UN is not going anywhere; they need to understand that the UN is an institution that has grown up and reached its full potential, which is to say, not very much at all. The UN is what it is; and it will neither disappear nor fulfill any grand dream of liberal internationalist global governance.
And, Michael Schmitt, Legitimacy Versus Legality Redux: Arming the Syrian Rebels (2013):

This article examines the international law issues surrounding the US policy decision to arm Syrian rebels. Topics discussed as potential violations of international law include the prohibition on the use of force, the principle of non-intervention, Security Council action and State responsibility for any unlawful activities of the rebels. The Article also examines possible justifications for the action under international law including self-defense, military aid to a government, humanitarian intervention, an action against the enemy during an armed conflict, and the taking of countermeasures. The article concludes that arming the rebels is questionable as a matter of law, although it notes that it may be legitimate (it draws no conclusions on this latter point).
Regards

Mike