Now Obama must act to defend US credibility
More than two millennia ago, the rulers of Carthage tested the resolve of a superpower by attacking the neighbouring kingdom of Numidia.
It was an unwise decision. Rome responded by launching the Third Punic War, reducing Carthage to ashes.
The lesson was that goading a superpower can be a dangerous business.
In the end, a superpower's most valuable possession is credibility. If the world's pre-eminent nation makes a threat, offers a guarantee – or draws a 'red line' – it must be prepared to enforce its will.
Only last week, he dwelt on the risks of "being drawn into very expensive, difficult, costly interventions that actually breed more resentment".
America might be the focus of global expectations, but that "does not mean that we have to get involved with everything immediately", he said.
He added: "We have to think through strategically what's going to be in our long-term national interests."
And that is why US military action against Syria now seems likely. If the Cruise missiles are launched, they will not be dispatched because Mr Obama has been converted to the cause of intervening in Syria. Nor will he necessarily have decided that America must topple Assad by force.
No, if this campaign begins, it will happen because the president feels he has no choice. The credibility of the US – a "long-term national interest" par excellence – is at stake, and no leader of a superpower can allow that particular asset to be jeopardised.
On August 20 last year, Mr Obama drew his red line, telling Assad that any use of toxic gas would, "change my equation". Yet the signs are that Assad's regime has poisoned its enemies on a scale unknown since the days of Saddam Hussein.
What sticks in the mind is the wanton recklessness of this atrocity. Chemical weapons were not merely used, but deployed to kill hundreds of people. The chosen targets were only a half-hour drive from the hotel where UN inspectors happened to be staying.
The attacks were timed for almost the actual anniversary of Mr Obama imposing his red line. And, most remarkably of all, this terrible weapon was brought out from the arsenal at a moment when the war had actually swung in Assad's favour.
The president will have been driven to ask: how can America's credibility be preserved if I do not respond?
At this very moment, a range of terrible things are not happening largely because of the power of America's word.
North Korea is not invading South Korea; China is not laying its hands on chains of disputed islands; Russia is not threatening the Baltic states. And, of greatest relevance to Mr Obama's decision over Syria, Iran is not racing to achieve the ability to build a nuclear weapon.
By his own folly, Assad will have brought America's military might down on his head.
Just as the hapless leaders of Carthage underestimated the resolve of Rome, he will have forgotten that the authority of a superpower's word is the one prize that any US president must, if necessary, defend with force. Otherwise, he would no longer be leading a superpower (© Daily Telegraph, London)