It is not so much the crossing of a red line but the tipping of a balance which has triggered the need for a Syrian intervention. MPs must prepare to send Britain's military into action once again.
Bashar al-Assad must prepare for retribution. It's a satisfying sentence to write because it's rooted in emotion. From John Kerry's talk of the "moral obscenity" of chemical weapons to David Cameron's spokesman talking this morning of their "completely abhorrent" use, so much of the debate around a western strike against Syria is all about fostering moral righteousness.
It becomes a question of humanity. The international community has wrung its hands over the Syrian civil war for months but now finds itself confronted with a new level of horror. Intense and widespread suffering is set to follow if there are repetitions of what appears to be the world's most lethal chemical weapons attack since the 1980s. This is the cost of inaction for the leaders of the west.
However compelling the emotional case is, it is not enough. There must be clear pragmatic reasons for any armed intervention. And in this case there are. A series of missile strikes, the most likely form of an attack, stand a good chance of pushing Assad towards talks with the rebels.
They will never prove decisive either way - such an outcome is simply not within the power or reach of the west. But it could change the equation and put Assad back in his box. For the desiccated pragmatists of foreign policy, this would count as a significant success.
Critics of the warmongers might find themselves influenced by Iraq. This would be a mistake, for Syria is never going to be about 'boots on the ground'. All the objections based on fears that this will be another Iraq or Afghanistan are looking in the wrong history books. Libya is a more obvious parallel. The Nato strikes on Serbia in the late 1990s are another. Both carried military risk, but the scale of the challenge was much smaller.
More than both the emotional and logical reasons for an intervention, though, comes the decisive factor.
This is the question of credibility. The Obama administration had made clear there would be consequences if the "red line" of a chemical weapons attack was crossed. Now that line has been crossed and, if the US and the west are to halt their grim retreat from the Middle East, consequences must follow.
It was not a mistake for Obama to mention those red lines, because the use of chemical weapons against civilians does send the conflict towards an entirely new level of suffering and death. Now the rhetoric has committed him to action it is time for that action to take place.
A failure to do so would be viewed as an admission of weakness from the rest of the world. That doesn't just mean the Arab world but the global power players like Russia and China which monitor western resolve with the closest of attention.
Any such move will irritate Moscow and rile Beijing but these countries have long appreciated their isolation on the issue of intervention. Their discomfort at Libya will be magnified and some will warn of the development of a proxy war. That seems excessive, for while kicking up a fuss they will realise that there is only so much they can do to prop up Assad against the damning revelations of chemical war.
There are risks in terms of collateral damage. There are always dangers of political mission creep, too. It is up to the politicians to handle these better than they have in the past. They have to be weighed up against the dangers of inaction - of many more horrible deaths in a never-ending civil war, all taking place in a region increasingly unconvinced of the west's resolve to stand up for what it believes in.
A controlled set of military strikes has to be the answer. The balance has shifted, and this Thursday MPs should decide the time has come to act.
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