In some conflicts, outside powers can be moved by a catalytic event, even after weeks or years of standing on the sidelines, to step in and change the balance of forces. That's what happened in 1991, when heart-rending footage of Iraqi Kurds fleeing over snowy mountains to escape the vengeance of Saddam Hussein forced President George H Bush to impose a no-fly zone.
It happened again in 1995 when the Srebrenica massacre of Bosnian Muslims transformed Bill Clinton from a diehard non-interventionist to a believer in the use of US military might to bring the Bosnian war to an end.
The massacre in Ghouta just outside Damascus on Wednesday morning looks like one of those transformative moments. The ranks of the dead and injured were captured in appalling detail in smartphone video. They all showed the victims - including a large number of women and children - of what appeared to be asphyxiation from gas.
William Hague, the British foreign secretary, called for the UN to mount an immediate investigation. He said the attack should "wake up some who have supported the Assad regime, to realise its murderous and barbaric nature".
His appeal fell on deaf ears in Moscow, the main backer of Bashar Al Assad's regime. The Russians called the event a "planned provocation" by the rebels - meaning the rebels had faked it, or massacred their own people, in a gambit to drive President Barack Obama to intervene with force.
It was almost exactly a year ago that Mr Obama said the use of "a whole bunch" of chemical weapons by the Syrian army would be a "red line". Since then the US and its European allies have concluded that the Syrian armed forces have indeed used some sarin, a nerve agent, but the White House made clear that this had been on too small a scale to warrant a radical policy change.
Not surprisingly, on Wednesday evening the United Nations Security Council remained stuck in shameful immobility. The Russians, with Chinese backing, rejected a call for an immediate investigation. The Council could go no further than to make a limp appeal for "clarity".
With no immediate prospect of a proper scientific investigation before the evidence disappears, one thing is clear: the footage of the dead, dying and injured is not an elaborate fake. Though the casualty figures are not clear, it is beyond doubt that, in the words of one chemical weapons expert, "something terrible happened".
On the basis of the footage the killer chemical does not appear to be sarin, but it could be other non-military chemical such as a pesticide or riot gas. One explanation is that the victims were sheltering from army bombardment in basements, where a harmful gas could easily reach a fatal level of concentration.
The type of chemical agent is an academic question, and to dwell on it too long suggests foot dragging. The deliberate killing of large numbers of civilians is a war crime that should be investigated.
Meanwhile, debate has focused on why the Syrian armed forces should launch a chemical attack in the outskirts of Damascus while a team of UN chemical weapons inspectors is staying in the capital, just 15 minutes drive away, with a mission to investigate past allegations of the use of sarin. Some media outlets, without buying the Russian line, have described the timing as suspicious.
To a rational outsider, it does seem reckless, but it is not unthinkable. Though the regime has had some success in the centre of the country around Homs, the rebels are dangerously close to the capital, and all past offensives have failed to move them back. Indeed, there are reports of the rebels gaining ground. This is sufficient cause for the army to use its ultimate weapon.
From the point of view of the Assad clan, Mr Obama is a busted flush. He has blurred his own "red line" - which is more damaging to US credibility than never having drawn it. With Moscow's diplomatic support, and Iranian military backing, they may believe they have carte blanche.
If there was any doubt as to America's lack of stomach for a new war, it has come from General Martin Dempsey, chairman of the joint chiefs of staff. In an extraordinarily frank letter, Gen Dempsey wrote that he opposed even limited intervention in Syria. The opposition was divided and if the Assad clan was removed, the victorious rebels were not likely to pursue US interests.
To judge by the low level of the Obama administration officials appearing on our TV screens, Mr Obama does not want this to be the kind of catalytic event that forces an abrupt change of policy.
If the past is a guide, he will think long and hard and, as ever, domestic politics will be at the forefront of his mind. He believes that Americans do not want and should avoid any new military entanglements in the Middle East.
While the military balance of forces may not be about to change in Syria, this massacre will weaken Russia's diplomatic position.
If the Russians are so sure that the massacre is an elaborate piece of stage management by the rebels, then they should be in favour of a quick UN investigation. What is stopping the Kremlin demanding an inquiry that will clear their client?
On the other hand, if Moscow continues to stand in the way of an investigation there can be only one conclusion: they know their ally is guilty as charged and are covering up a war crime. The weakness in the Russian position is something that should be aired in public during the G20 meeting in St Petersburg next month.
On Twitter @aphilps
