At what point did Bashar al-Assad cross Obama's red line? Was it the moment his forces deployed sarin gas against civilians? Or more likely, the moment his troops, with Hezbollah re-inforcements, flushed rebels out of the town of Qusair last week.
America, Britain and France see alarming signs that the regime is winning. Or at least, not losing.
- What incentive is there for Assad to enter peace talks while his men are advancing on Homs, Hama and Allepo?
- What are the odds of Assad agreeing to rebel preconditions for peace talks, that he should step aside?
- What chance the fractured opposition coalition would go into talks knowing its military position is weak?
Obama says the use of chemical weapons changed his calculus. The calculus that he really wants to change is Assad's increasing confidence that he can survive.
The weapons might be enough to draw Syria's opposition in exile into a peace conference delayed and re-scheduled, more in hope than expectation, for July.
Perhaps the regime will also turn up. It depends on the pressure Russia can exert.
But even within the American administration there is the realisation that US supplied assault rifles might be too little, too late to change the brutal dynamics of the battlefield.