The First Communicater explains the principle behind a military attack on Syria:
“Gassing innocent people, delivering chemical weapons against children is not something we do,” Mr Obama said at a post-summit press conference. “It’s prohibited in active wars between countries, even more so against children, and we’ve got to stand up for that principle.”
Syria is engaged in a civil war, not an "active war between countries". I don't have the background of a Smart Diplomat but it seems that the 1925 Geneva Accords banned the use of chemical weapons between warring countries and was silent on their internal use. However, norms have evolved and subsequent treaties now ban the production and stockpiling of these weapons, which makes their use in any context a treaty violation, since you can't use what you can't have.
The use of chemical weapons is very clearly prohibited in international law under a number of different legal instruments. Syria is not a party to the 1993 Chemical Weapons Convention (CWC), which unequivocally prohibits chemical weapons use (and other activities) under any circumstances. However, in 1968, Syria acceded to the 1925 Geneva Protocol, which prohibits the use in war of asphyxiating, poisonous or other gases, and of bacteriological methods of warfare.
Syria’s adherence to the Geneva Protocol may have led the Assad regime to think twice about using chemical weapons against Syrian civilians – in what has now been declared a ‘non-international armed conflict’ by the International Committee of the Red Cross (ICRC) – even if the treaty was originally intended to cover international conflicts. The ICRC concluded in 2005 that customary international humanitarian law includes a ban on the use of chemical weapons in internal as well as international conflicts, and an appellate chamber in the International Criminal Tribunal for the former Yugoslavia (ICTY) noted in 1995, in Prosecutor v. Tadic, that ‘there had undisputedly emerged a general consensus in the international community on the principle that the use of chemical weapons is also prohibited in internal armed conflicts’.
Which is fine by me. And if I were explaining the principle by which Syria was violating international norms I would not include a puzzling qualifier about "active wars between countries". Chemical weapons are evil and the world has banned their use - is that too complicated?