Conducting interventionist wars is wrong. Whether you agree with that statement or not - and, while I'm confident in my position, there are arguments on both sides - you have to admit it's a statement about rightness and wrongness, not about the law. That is, it's an ethical claim, not a legal one. You might mount a detailed, sourced and accurate argument that interventionist wars, under certain circumstances, are not illegal and are in fact mandated by certain provisions of international law. Spend ten minutes on any political blog's comment section and you'll see a hundred examples of this kind of argument. However, the law is totally irrelevant to the question of whether we ought to use military force (read: kill people) in Syria etc. I think that the irrelevancy is obvious, but I'll argue for it anyway.
Here's the obligatory argument against using the law as a basis for morality. Unlike ethics, which in theory proceeds from empathy, rational thought, conscience and so forth, the law proceeds from political considerations. Even if we conceive of the State as a wholly benevolent entity (which, as an anarchist, I do not) the law is still not a guide for ethical action, but rather a guide for how a far more powerful third party might force people into acting as ethically as possible. That's why we don't have laws against being rude or lying - not because those things aren't wrong, but because it would be wrong to try and enforce politeness with the police. If we conceive of the State as a partially-benevolent or minimally-benevolent entity, then using the law as a basis for morality is laughable. Here are some reasons why, each of which is decisive:
1) Unethical laws exist in every real-world State. Segregation, protectionism, criminalization of things that should not be criminal. If the law was a basis for morality, this trivial statement would be incoherent.
2) Seeing the law as inherently moral gives us no way to fight back against and change bad laws - which is what defenders of the law generally want people to do.
3) The process of how law is formed is transparently influenced by corruption, compromise and political consideration, none of which are appropriate ways to form ethics.
So while the legal system might help us judge what we ought practically to do, if we wish to avoid the boot and the Taser, it's poorly suited to help us judge what we ought morally to do. The larger question, however, is this: why do people constantly bring legal considerations into discussions of ethics?