How will the world respond to suspected war crime in Syria?

French Foreign Minister Laurent Fabius said that outside powers should respond “with force” if UN officials confirm that Syrian forces mounted such an attack. Foreign Minister Ahmet Davutoglu of Turkey said “all red lines have been crossed”, and that if outsiders did not act, they would lose the power to deter future Syrian government attacks.

Over the next few days or weeks, the suspicion that Syrian President Bashar Assad’s forces unleashed toxic chemicals may harden into evidence of a war crime. If so, that would – should – change the world’s calculation about how to help rebels overthrow Assad.

We know this much already:

The UN won’t be much help, despite resolutions frowning on chemical weapons. Russia, Assad’s chief enabler, blocked a UN Security Council statement condemning the attack. All the Security Council could manage was a call for a “thorough, impartial and prompt investigation”. Advantage, Assad.

Assad can defuse this crisis and prove that his government didn’t launch these attacks if he provides full access to the attack sites for UN inspectors to gather evidence. Anything short of granting immediate access will suggest to the world that he’s guilty. Let’s see what the dictator permits.

In any Middle East crisis, all eyes swivel toward the US. But gassing civilians is a war crime. It’s not just an affront to the US, but to humanity. So the reaction should come not only from Washington.

In June, the White House declared that Assad had crossed President Obama’s “red line” by using chemical weapons in smaller attacks. Since then, Assad’s regime, once near collapse, has grown stronger. The rebels are weaker. The Obama administration has dragged its feet on its June 13 promise to provide military assistance to selected rebel groups. The rebels say they’ve received no weapons or ammunition.

We understand the reluctance of Obama and other world leaders to get involved in Syria’s intramural warfare, no matter how much they dislike Assad: the rebel groups are increasingly fractured and jihadists are flooding the country. That means it is harder for the US and its allies to find moderate rebel leaders who would align with US interests.

Nevertheless, the world approaches a moment of decision. If the Syrian government launched this chemical attack, will it be held accountable, not just by the US but by countries in the Arab world and elsewhere? Will France, Turkey and other countries outraged by the attack muster the sand to impose a no-fly zone in Syria, along the lines of the Nato coalition that helped topple Libyan dictator Muammar Gadhafi in 2011?

Or will world leaders shrug, await Assad’s next outrage, and debate the meaning of “red line”?