We are caught between a rock, in the form of the recklessness of Donald Trump, and a hard place, shaped by the cruelty of Bashar al-Assad. This is the choice that now confronts citizens and their representatives in Britain, France and the US. The reasons to resist signing up for any project led by Trump should be obvious, with the newly published testimony of James Comey, the FBI director he fired, providing a fresh reminder.

Trump is a congenital liar who is devoid of empathy, a narcissist with a nihilist’s view of the world. These are not mere character defects; they have a bearing on the decisions the de facto leader of any action in Syria would take. Among the reasons I opposed the 2003 invasion of Iraq was my fundamental distrust of George W Bush and his circle, especially on the matter of motive. Trump, with his tweeted mood swings – first, vowing to withdraw from Syria altogether, then threatening an imminent missile bombardment, then signalling a delay – makes Bush look like a statesman.

But even if a moral paragon were sitting in the Oval Office, there would be grounds for restraint. The record of past western military interventions in the Middle East is bloody and shaming, as the peoples of both Iraq and Libya can testify. Barack Obama, no gung-ho cowboy, was the commander-in-chief in the latter case. And yet what was originally billed as a discrete military action to prevent an impending civilian slaughter in Benghazi escalated into a bombardment that led to regime change and mayhem. It stands as a textbook illustration of western bombs’ ability to make a bad situation worse.

Those warnings from the past gain extra force in the current case, because standing in the way of any allied operation would not just be Assad, but also the military might of a nuclear superpower. That makes the task this time all the more delicate, for any action would have to avoid triggering a military confrontation with Russia, whose forces are present on the ground and in the air in Syria. The notion of entrusting such a task to a man as reckless as Trump would itself be reckless, criminally so.

The natural response is to steer as fast as we can away from that rock, but before we know it we are crashing into the hard place. The notion of inaction, of standing by and watching as Assad kills and kills and kills, racking up a death toll in Syria of 500,000 and turning millions into refugees – that prospect too should sicken us. And yet that’s what we’ve done. For seven slow years, Assad has been allowed to play butcher, uninterrupted in his work as he cuts down the people of his own country, with barely a hand raised to stop him.

For the parent of a murdered Syrian child, the current western focus on the exact method of murder must feel strange. As if Assad was well within his rights to slaughter innocents using regular bombs, and his only offence was to use chlorine or sarin, inflicting a death so painful the footage is unbearable to watch.

It is indeed strange, but the extra revulsion at the use of chemical weapons is not groundless. The taboo on the use of such weapons held, with exceptions, for nearly a century. It meant there was a limit. If Assad’s crimes go unpunished it means a new norm will have taken its place, one that says dictators can gas their own citizens with impunity. Who wants to live in such a world?

So this is the spot we find ourselves in, caught between giving a blank cheque to Trump and giving a free pass to Assad. What other options are there?

Labour calls for the attack on Douma to be “fully investigated”. That sounds unarguable. But then what? Jeremy Corbyn issued the same call after the chemical attack that killed at least 74 at Khan Sheikhoun a year ago: demanding there be a “UN investigation and those responsible be held to account”. The UN duly investigated and in October concluded unambiguously that the Assad regime had used sarin gas. But Corbyn greeted that verdict with silence. So unless there’s a plan for action once guilt is established, demanding an investigation sounds a lot like an excuse to do nothing in the hope that soon we’ll all be talking about something else.

Assad’s guilt is clear. Yet some treat each new attack as if it were the first

Besides, how much evidence do we need? Even before Douma, Assad’s use of chemical weapons had been documented seven times this year alone. To all but the most committed denialists and conspiracists, Assad’s guilt is clear. Yet some treat each new attack as if it were the first.

Nor will it do simply to call for diplomatic efforts or a political solution – though, of course, such calls sound admirable – as if there haven’t been years of diplomatic efforts, round after round of Geneva talks, the UN now on its third special envoy, all thwarted at every turn by Assad and his protector Vladimir Putin, wielding Russia’s UN veto. Calls for talks or investigations might sound like cries for peace: in this context, they are pleas for Assad to be allowed to keep on killing in peace.

Are there no good options then? I can’t see any. But perhaps the least bad comes from a voice we hear rarely, that of the democratic Syrian opposition and the groups which represent Syrian civil society, now scattered and in exile. The Syrian Negotiation Commission has called for action to deter Assad from killing civilians. What they envisage is that each time Assad launches a deadly attack on noncombatants, allied forces reply by taking out one of the strategic assets he uses to kill civilians. It could be an airfield, it could be a command centre. If the target were aircraft, that would simultaneously inflict a cost on the regime and deprive it of the means of dropping its barrel bombs and toxic, yellow cylinders. The objective would be to make Assad pay a price for killing his own people, a price he has not paid until now. Eventually, or so runs the hope, he would be deterred.

In other words, not an all-out bombardment, not an invasion, not regime change, not a re-run of Iraq or Libya. A methodical, tightly focused attempt to deter the Assad regime from killing civilians and robbing it of the ability to do so. Given Russia’s presence, it would not be easy. But this is what Theresa May should be proposing to Trump and to Emmanuel Macron. Indeed, she should make UK support for any military action conditional on it being the right kind of action. In a parliamentary debate – which Labour MPs have every right to demand on a question of this gravity – MPs should impose the same conditions on May.

Two decades ago, the world watched in horror as slaughter unfolded in the Balkans. In the end, thanks in part to western bombs, the perpetrators were forced to stop the killing and come to the peace table. Once backed by the threat and use of force, diplomacy was able to work – but only then.

Of course, a bigger memory has eclipsed that one. The deadly cost of Iraq haunts us, as it should. We saw the havoc western action could wreak. Perhaps that was why, five years ago, the House of Commons voted to leave the Assad regime untouched. Back then the death toll in Syria stood at around 100,000. More than 400,000 have died since that day. The proof is there if we can bear to look at it. Inaction, too, can be deadly.

• Jonathan Freedland is a Guardian columnist