Well perhaps they would set up an extra-governmental agency, to which all UN member nations would donate funds and military hardware and personnel. This agency would not be under the control of any single nation, or even a group of nations, it would be fully independent. It would be tasked with examining allegations of chemical weapon use, and with carrying out punitive strikes on any nation that used chemical weapons.
Yes, including the USA.
In short, the community of nations would set up a Leviathan to enforce the laws they had agreed upon. The current environment of anarchy at the highest levels is a complete clusterfuck.
I doubt very much any nation actually cares enough about chemical warfare to even suggest such an action. They much prefer the option of responding only to those infractions that it is personally advantageous for them to respond to. Chemical weapons are much too valuable as an excuse – an outrage to rally the people around – to actually get rid of them!
Stross has a brilliant idea – let’s take the money we would be using to kill Syrians and use it to save them instead!