In both cases, the United Nations seems the best vector, as it has inspectors on the ground and its Security Council would provide approval for a strike.
But the inspectors, in a preliminary report likely to be released over the weekend at the earliest, may be unable to say whether it was the regime or the rebels who carried out the attack.
And Russia, a Syrian ally and arms supplier, wields the right of veto at the Security Council - as does China, traditionally resistant to any action that infringes national sovereignty.
The UN may be the first port of call for the Europeans and the Americans, but if this fails, their case is far from lost, say some analysts.
On the challenge of presenting a moral argument, countries are already stoking a sense of outrage, they note.
Australia, Britain, Canada, France, Germany, Italy, the Netherlands, Spain and the United States have expelled Syria's senior-most diplomat to their country in a reaction to the attacks.
As to what happened on the ground, the case against Syria is far stronger than it ever was against Iraq, say these voices.
"The problem in Iraq was that [former President] Saddam [Hussein] lied to his generals, and so both defectors and regime officials whose conversations were spied upon believed they had WMD," says former Pentagon official Michael Rubin.
"In Syria, the body count confirms use of chemical weapons."
As for the UN seal of approval, Rubin is dismissive.
"The UN lost credibility decades ago. Just take a look at the Rwandan killing fields, or ask the Kurds in Halabja," a town where several thousand were gassed to death by Saddam in 1988.
"I don't recall Russia seeking UN permission when they invaded Georgia," in 2008, he added sardonically.
Lacking explicit Security Council endorsement, countries could invoke a principle in international law called the Responsibility to Protect.
"There is certainly plenty for the politicians to play with in order to create a legal cover story to their own satisfaction," said David Butter, Middle East expert at the London think-tank, Chatham House.
Known by its acronym of R2P, this is a principle of holding states responsible for the protection of their citizens and permitting outside intervention on humanitarian grounds.
Introduced in 2005, the concept is gaining prominence, even if it remains rather woolly in definition.
It requires an atrocity to be a current threat; for diplomatic and other options to have been exhausted; and for any intervention to be limited to protecting the civilian population.
Neither British Prime Minister David Cameron nor French President Francois Hollande have directly referred to R2P to justify an attack on Syria, but both seem to be preparing the ground for invoking it.
Cameron has said any response "would have to be legal, would have to be proportionate, it would have to be specifically to deter and degrade the future use of chemical weapons," while Hollande has insisted that any action would be "appropriate".
"Basically, R2P has become an excuse to bypass the Security Council," said Rubin.
Beyond the concepts of morality and law, the anti-Syrian bloc can also draw on a decade of experience in dealing with the practical challenges of this kind.
Kosovo in 1999 and Afghanistan in 2001 marked Nato unity in tackling rogue or broken states.
But consensus fell apart in 2003 when many states were unimpressed by the evidence to strike Iraq.
This led the United States, Britain and other Nato members to assemble a "coalition of the willing" that toppled Saddam.
"The development of coalitions of the willing is something we have been seeing for a long time," Jan Techau of the Carnegie Endowment for International Peace told the Herald.
"We have countries prone to intervention and the ones who want to stand on the sidelines. Syria is not going to drive Western countries apart."
