'We have been very clear to the Assad regime... that a red line for us is we start seeing a whole bunch of chemical weapons moving around or being utilised.
'That would change my calculus; that would change my equation.'
This was a clear threat to repeat the 2011 Nato assault which resulted in the overthrow and murder of Libyan leader Muammar Gaddafi.
So what is the evidence that Assad recently chose to do the one thing most likely to trigger a Western attack and similar fate?
On April 25, the White House claimed that US intelligence assessed 'with varying degrees of confidence' that 'the Syrian regime has used chemical weapons on a small scale in Syria, specifically the chemical agent sarin'.
Having offered this caveated assertion, US Secretary of Defense Chuck Hagel added:
'We cannot confirm the origin of these weapons... but we do believe that any use of chemical weapons in Syria would very likely have originated with the Assad regime.'
'As I've said, this is serious business – we need all the facts.'
A sceptical Alex Thomson, chief correspondent at Channel 4 News, commented:
'WMD, the Middle East, and here we go again... Already a British prime minister is talking about a "war crime" whilst offering the British people no detailed evidence.'
Evidence included video footage said to show victims of chemical weapons foaming at the mouth.
Thomson offered a link to a detailed report of the 1995 sarin attack in Tokyo, noting: 'am advised there's no mention of any prominent bright, white foam at mouths'.
Thomson also asked, reasonably: 'Why doesn't any medic in the film wipe away the white foam on patients' mouths – the basic paramedic fundamental to preserve an airway?'