According to several reports, chemicals weapons have been used by the ruling regime in Syria to attack the rebels who are fighting them. Chemical weapons are banned by several treaties, and for good reason, so if they have been used it certainly a breach.
However, there is no current certainty about the precise details of the attack, as UN investigators are on the ground trying to work out what has happened. Even if they find some evidence of it though, Russia and China are still likely to veto any action against Syria.
So, should the US, the UK and other allies act without a UN mandate on humanitarian grounds or wait until a UN Mandate is given.
And if they do act, how much action should they take?