Let’s cut the canting. No one thinks, not those ordering them and not those opposing them, that the missile strikes against the Assad regime will influence the outcome of the catastrophe in Syria. That pass was sold long since. If there was an opportunity for America, Britain and their allies to make a difference for the better, the chance was missed many, many deaths ago. What we are witness to – on the part of both the leaders of the western democracies and their critics – is a tableau of actors striking postures designed to make the players feel better about themselves. This posing can never rewrite the blood-drenched history of a seven-year conflict that has turned Syria into a charnel house and shredded international norms about the conduct of war.

The proximate cause of this crisis is the chemical attack on Douma last weekend. After years of unmasterly inactivity by the democracies, it is that atrocity that drew attention back to what is happening in Syria and finally stirred punitive action against Bashar al-Assad. In the words of the ineffable Donald Trump, the retaliatory strikes are supposed to demonstrate to “animal Assad” that there is a “price to pay” for the dictator’s use of banned weaponry. In the more measured language of Theresa May, “we cannot allow the use of chemical weapons to be normalised”. Yet the normalisation of chemical weapons is precisely what has already happened in Syria. Assad’s regime has time and again used chemical warfare to slaughter its own people, as it has also deployed hideous “conventional” weapons such as dropping barrel bombs and fuel-air bombs on civilian areas to inflict mass casualties.

Over seven years of relentless savagery in Syria, the hands of the leaders of the western powers have been wedged firmly under their bottoms. They have been encouraged to maintain this impotent posture by legislators too feeble to grip the dilemmas posed by Syria and voters weary of engagement with the hard parts of the world. Listening to both their public pronouncements and their private calculations, the abiding impression is that this belated and limited action by Washington, London and Paris is not driven by any conviction that these strikes will make any meaningful difference. They won’t even stop Assad manufacturing more chemical weapons if he chooses to do so. Missiles are flying mainly to soothe guilt about repeated earlier failures to act.

Even so, I give these leaders a little more credit than I can find for those whose only counsel is to do absolutely nothing. At least some of these belated interventionists are wrestling with a genuine dilemma. Action over this chemical attack, when so many previous atrocities have been given a free pass, looks like therapy by Tomahawk, a gesture, not a strategy. But to let yet another use of chemical weapons happen without any form of response would have given a complete sense of impunity to the Assad regime and its sponsors in the Kremlin. Every dictatorship on the planet has been getting the message that there is no penalty for the acquisition and use of weapons prohibited since the First World War and that has chilling implications for future conflicts.

The other side of our tableau of posturers is composed of those who oppose this action as they have clamoured against all previous attempts to do something about Syria. The non-interventionists come in two categories. There are the “it’s nothing to do with us” brigade who declare that “we haven’t got a dog in the Syrian fight”. Mainly to be found on the hand-washing right, the cold brutality with which they express their indifference to so much human suffering has the sole merit of being candid.

Less honest, not least with themselves, are the self-proclaimed peace-lovers. Mainly to be found on the hand-wringing left, they are too busy looking in the mirror admiring their own halos to face the moral challenges posed by a situation like Syria. Jeremy Corbyn opposes this weekend’s action on the grounds that it “risks escalating further” what is “an already devastating conflict”. The Labour leader and those who share his worldview are consistent. Do nothing has been their unvaried policy for the past seven years of carnage. There is no doubt that they can expect support from a lot of a domestic electorate turned allergic to engaging with abroad, especially the Middle East.

While the non-interventionists have talked about talking, the Assad regime and its backers have been busy killing

As the non-interventionists have preached inaction, the death toll in Syria has been remorselessly – what’s the word, Jeremy? – escalated by the Assad regime and its allies. Whenever pressed to say what they would do, the non-interventionists fall back on calling for “negotiations” and more effort at the United Nations. They have to be aware that Russia has repeatedly used its security council veto to shield Assad from any effective action by the UN, most recently by blocking an independent investigation into the chemical attack on Douma. While the non-interventionists have talked about talking, the Assad regime and its backers in the Kremlin and Iran have been free to go on killing.

Some argue that the fatal turning point was the summer of 2013 when Assad’s use of chemical weaponry defied a “red line” drawn by Barack Obama, who then failed to enforce it. That has become the main reference point of the British argument because it was then that David Cameron failed to secure parliamentary approval for participation in punitive air strikes. When MPs turned him down, the then prime minister abandoned any further effort with a huffy “I get that”. The people who really “got it” were the Syrian civilians who continued to die in their thousands.

It is possible that intervention at that point might have made some difference. In truth, the west made its gravest decision not to act earlier when Assad responded with vicious suppression to the original uprising against his dictatorship in 2011. Rather than offer succour to the secular democrats who rebelled against the regime, the west stood by as he imprisoned, tortured, executed and crushed those who cried out for freedom. Every force in Syria was provided with resources and weapons except for the liberal opposition. Humanitarian initiatives could have been attempted, such as creating safe havens for civilians and imposing no-fly zones to impede Assad’s ability to slaughter his own people. They weren’t even tried. No meaningful pressure was applied to bring him to the negotiating table. The opposition become dominated by jihadists. Regional powers and Russia moved into the voids left by western inaction.

It is not provable whether earlier intervention would have altered the course of Syria’s tragic history. Non-interventionists said then, as they say now, that anything that the west does only makes things worse. That we can’t prove either. What we can see is how bad things have become and it is hard to conceive how exactly it could be worse. After seven years of failing to act in Syria, we can audit where a non-interventionist policy has got us. It has been an utter disaster in every respect.

The United Nations struggles to put an exact figure on how many people have died, but best estimates put the number at around half a million. More than 5 million Syrians are refugees abroad and more than 6 million have been internally displaced. Assad has flattened cities and smashed through nearly every international taboo about the conduct of war. The Syrian dictator is massacring his way to victory and there is no one who thinks that this weekend’s missile strikes will in any way impede him. The conflict has inexorably widened and now consumes the region as Syria has been turned into a battlefield for proxy conflicts between regional players. Russia has been encouraged in its belligerence. The west looks helpless. Dictators the world over have been emboldened to believe that they can crush opposition using the most barbaric methods and the rest of the world will do nothing to stop them. Those striving for freedom have been commensurately disheartened. The rule of international law has been weakened.

Action has consequences and they are not always the ones intended and hoped for. That was the grisly lesson of Iraq. Inaction also has consequences. Doing nothing can have a price every bit as high. I’d think better of the non-interventionists if they’d ever once admit that. Inaction has been a terrible choice in Syria.

Interventionists have been rightly obliged to own all that went horribly wrong in Iraq. Non-interventionists, the horrors of Syria are on you.

• Andrew Rawnsley is an Observer columnist