So let's just assume for this discussion that Assad's army indeed used chemical weapons in Syria and that it was not as alleged by Assad's regime used by the rebels.
The usual way to go about this is to call the UN-security-council and use a resolution to condemn Syria for it and to decide upon sanctions, be it economical or military ones.
Legally states have souvereignity and so as long as a state doesn't attack outside of its country, no other country is allowed to interfere.
After ww2 five states who have atom-bombs assumed an authority that is above the souvereignity of states. When these five atom-bomb-states are in agreement, they can nullify the souvereignity of a state and order an intervention in the name of restoring or defending world-peace/security.
Then there's of course the question of the use of abc-weapons. Internationally there are laws against the use of them, but has Syria signed and ratified any of these. If yes, then Syria could be condemned and sanctioned upon these through the security-council.
The idea was to prevent another worldwar.
But when not in agreement, ie. when one of the five atombomb-states uses its veto-right, souvereignity of a state can't be breached and action against it can only be used in the usual defending one's own state, ie. when and if the neighbour attacks.
In Lybia a UN-resolution was used to weaken Gaddaffi's regime untl the rebels could win.
In Iraq an older UN-resolution was used as an excuse to intervene.
In Afghanistan the US made the case that 9/11-terror-act was committed by Al-Kaida, and that Afghanistan were harboring Al-Kaida or allied with it, so that attacking Afghanistan would be allowed under self-defense.
But in the case of Syria there seems to be no legal instrument to use to intervene.
In that instance it seems to be a similar case to the Kosovo-conflict, where the Nato intervened to stop the ethnic cleansing of albans through Serbia's government.
The Nato made the case that as a regional unity it can decide to intervene when regional stability is at risk.
There are proponents and opponents of that argumentation. Opponents would usually state that even a regional unity like the Nato needs an authorisation through the Security-council except for the case when a Nato-member was attacked.
If one would try to reproduce that model of regional-unity-intervention in the case of Syria, then only the neighbours of Syria could do it, ie. only the arabic council.
What do you think?