Even Assad's regime used those chemical WMD,then also USA do not have any right to attack Syria because Syria have'nt broken any international Law.The law by UN which governs the use of chemical weapons states that no chemical weapons should be used against any other country.It does not states anything about using chemical weapons against rebels or protesters.
When I posted my "no" answer it was earlier on in the Syrian Civil War.
Things are different now. ISIS is a new Taliban. They can not be allowed to get permanent control of a state. They need to be stopped.