Gentlemen--

Under US law a treaty ( and the UN Charter is a treaty) has the same status as a statute passed by Congress and signed by the President. It is superceeded by the next law on the subject. An authorization for the use of force passed by Congress, a use of force under the War Powers Act, use of force under the inherent powers of Article II of the US Constitution all override the UN Charter for the US and therefore are legal under US law.

More important, a law that is not enforceable is not a law - at least in analytical terms. International law is never enforceable without the the express consent of the states concerned (even if after the fact of military defeat). Of international institutions, only the UN Security Council has the authority to enforce international law (plus the power to do so) with the caveat that it must not have the formal opposition of a permanent member of the UNSC and it must have a total of 9 positive votes. If this does not happen then the UNSC is unable to act. It does not make action by others impossible but only without UN sanction. is such action illegal? I am sure that many international lawyers would argue it is. But I return to the point I made in the first sentence of this paragraph: A law that cannot be enforced is not a law. Liewise, a law that is not enforced is not a law.

As far as chemical weapons are concerned we have the Geneva Convention (Protocol?) of 1925 banning them and the more recent chemical weapons convention. These are the relevant "laws" on the books. If neither the UNSC nor any state or group of states chooses to enforce them, then they are no longer law and we are back to the international anarchy among states of 1648 - which we really never left, only mitigated.

Cheers

JohnT