We have allowed the idea of moral causes to be soiled

It is easy enough to grasp, meanwhile, that what is taken to be public opinion was given a curative dose of Tony Blair a decade ago. We won't be fooled easily again. Those of us who shouted as a minority when the Iraq outrage began can hear the muttering of a convinced majority now.

Mr Blair soiled the idea of moral causes, of liberal intervention, of civilisation's responsibilities. He taught the majority on these islands to see games within games. A massacre has taken place in Damascus? Was Parliament recalled because of the latest slaughters in Iraq? Because of mass murder by the Egyptian army? How does Libya fare, these days?

The fact remains that a massacre has been carried out in Damascus. Once we have told ourselves that "the West" has developed a suspicious urgency after two years of lassitude, that this is a proxy war between Saudi Arabia and Iran, between America and Russia, between the old French colonial interest and Turkey's new assertiveness, the ugliest of facts remains.

Tutored by Mr Blair, you can remind yourself that a government reaching for the Cruise launch codes was determined not to wait for even a preliminary report from UN inspectors. You can ask why the Assad regime, brutish or besieged, demented or depraved, would launch a chemical attack within its own capital just when those inspectors were arriving.

You can then spend plenty of time wondering about the membership of the "rebel alliance". Some of them are not pure in heart, mind or method. Some of them have precious little interest in the fate of the people of Syria. Our enemy's enemy is not necessarily our friend. And what qualified Bashar al Assad as our latest enemy? The fact that he is the figurehead of a dictatorial dynasty? Let's apply that criterion to our freedom-loving allies in the Gulf states.

Press on a little further. The spectacle of Washington, London and Paris promoting armed revolution is, let's say, a new one, historically speaking. The prospect of another Palestinian intifada has not exactly roused Mr Cameron's inner Churchill. Martin Luther King, had he witnessed Barack Obama's risible attempt at a commemorative impersonation, might have had a few words to say about universal equity and double standards.

These arguments are not hard to come by. But a massacre has taken place in Damascus. Those familiar with the filthy substances employed - and familiar for all the wrong reasons - could spot nerve agents at work in the bodies of children without an instant's hesitation. "War crime" does not cover the case. Here was a crime against humanity, a new order of atrocity, a nadir. Now what?

The least respectable argument arises from the self-serving nonsense of the "war on terror". By this piece of bastard logic we must throw in a few Cruise missiles lest "they", once nominated, do something like that to us. The second-worst case flows from the pretence that there can be a law higher than international law. Mr Cameron is outraged or Mr Obama is appalled. That, supposedly, is good enough. Mr Blair has been employing the excuse for a decade.

Yet the alternative is equally grisly. Legality, the responsibility to protect, the concept of an international community: all of that is derived, ultimately, from the deliberations of the UN Security Council. Russia has a veto, like the rest of the favoured few, within that body. So are we to allow Vladimir Putin to decide what is and is not legal?

Let's say that America's missiles are on their way. This time, Britain need not go to the bother of pretending to join in. For the White House, that's the loss of a paragraph in a press release. For Downing Street, it's a chance to worry that the French have left HMG looking limp. But a bombardment with the usual, inevitable killing of civilians will not prevent another chemical atrocity. It might even encourage a repetition. Then what?

All the armchair general stuff and the Westminster posturing has already begun to distract attention from what took place in the Damascus suburbs. It returns us to the one decent (in any sense) argument that Mr Blair managed to retrieve from the Iraq disgrace. This: just because you cannot prevent every atrocity, that doesn't mean you should ignore all atrocities. The use of chemical weapons in one place gives licence to depravity everywhere. If it is unchallenged, a precedent is set.

The West is hypocritical, unjust and self-seeking: true enough. Its people have been lied to once too often. Had it not been for Blair's deceits, "public opinion" would be demanding military action, after proof, against Mr Assad. But thanks to the People's Tony we are incapable even of assessing our impotence. Missiles don't work; the UN doesn't work; gung-ho Prime Ministers are a menace. Yet chemical weapons are loose in the world.

As usual, the infallible answer will not be found here. One thought might be a start, however. Why doesn't Mr Obama announce that he wants to call on Mr Putin in Moscow, that he intends to show the Russian every last piece of evidence, and add that he will publish everything if a consensus cannot be achieved? If it helps, the American President could take Mr Cameron, Francois Hollande and Angela Merkel along for the trip. The Chinese would probably appreciate an invitation.

This would "circumvent the UN", of course. Perhaps it's about time for that. Syria suffers, month after month, precisely because that institution has served its function by balancing the might of the great powers, one against the other. The deadlock needs to be broken. So recognise the reality of power and geopolitical "interests". Stop pretending to talk.

It would not inaugurate the kind of world in which I, for one, would want to live. It would make a mockery (another one) of international law. But it would have the virtue of honesty. Proxy conflicts have become all the rage in this 21st century. Westminster's little parliament has produced a mild surprise merely by resigning its usual role. But let's be plain: children have died horribly in Damascus thanks to rivalries among the dominant powers. Let the powers speak to that.

It won't happen, of course. That would give too much of the game away. Another part of the game is to bewail the fact that "we" have allowed chemical weapons to be used. I didn't; did you? Did an ordinary American, Russian or Syrian even contemplate the act? The atrocity exposes the myth of popular consent and control on which international law, the last shred of common decency, is supposed to depend.

Still, the horror has been committed. Most of us are innocent yet all of us, strangely enough, are indeed guilty. By the same neglect we allow nuclear weapons and liberty to deceitful former prime ministers. We are not to blame - how could we be? - but we are, far down the line, many miles from the carnage, at fault. And we don't know what to do about it.