Hand in hand, they emerged from the White House, and she has never quite recovered from it. The moment Donald Trump clamped his paw over Theresa May’s sent a collective shudder through not just viewers at home, but Downing Street staffers too. The prime minister went to Washington grimly determined to prove that whatever her personal feelings about Trump, the special relationship between the two nations endured, only to prove there’s such a thing as too special by half.

Trump says Syria decision imminent as Russia warns of 'grave repercussions' Read more

But now the limits of that relationship face a far more serious test. The US clearly intends to punish the Assad regime and its Russian backers for the atrocity in Douma, while stopping well short of anything resembling a ground war. And this time it doesn’t wish to be seen as going it alone, which means enlisting the practical or political support of Britain and France in bombing raids. Trump’s hand is on May’s sleeve again, forcing her to choose between an acquiescence that may turn stomachs at home and a rejection that would offend Britain’s most powerful ally.

Already siren voices are whispering that she can’t afford to lose any more credibility on the world stage, now Brexit has shattered our historic claim to being America’s hotline to the EU and vice versa. That it was France’s president, Emmanuel Macron, that Trump called first to discuss Syria is a clear sign we’re out in the cold. True, May could still command powerful allies’ backing in her showdown with Moscow over the Salisbury poisoning. But if anything, that creates an expectation of reciprocity. Hasn’t she just been arguing that the west must unite against Russian aggression, that the use of chemical weapons in Wiltshire threatened rules-based world order?

Yet if May is seen to be backing airstrikes just to curry favour with an increasingly erratic US, she won’t even carry a vote in parliament, let alone the nation. If it was hard enough for many to swallow the idea of standing shoulder to shoulder with George W Bush – imagine doing so with Trump as he blunders around the Middle East while blustering his way through the Stormy Daniels scandal at home. At least with Bush there was a possibility, however slim, of his heeding London’s advice at key moments in return for unflinching support. Who seriously imagines Trump in listening mode?

So somehow, May needs to turn this into a decision made strictly on merit, free from the ghosts of the past, rather than a referendum on transatlantic relationships. She should be very publicly weighing the risks of being sucked into a proxy war between superpowers or regional powers against the equally grave risks of allowing Russian aggression to continue unchecked or chemical warfare to be normalised. We need to hear about the prospects for a successful intervention that saves lives versus the risk of making another futile or counter-productive gesture that would cost them, not about how this affects Britain’s international clout. All of which means the specific (and so far unknown) details of how, when and where the US proposes to act are crucial. Beware anyone who claims the answers are obvious.

The underlying judgment is no easier now than in 2013. Military intervention can have terrifying consequences, but so can non-intervention. Calling for further inquiries to establish Syrian guilt just as previous inquiries into previous atrocities have done is likely, at best, to delay rather than remove the decision. Anyone arguing for a peaceful negotiated solution must explain how that is to happen when President Assad patently isn’t negotiating (why would he, with victory in sight?).

But what’s often lost in raking over that shameful night in 2013, when in a bewildering welter of amendments and counter-amendments in parliament stopped David Cameron authorising bombing raids on Syria, is precisely why it was shaming. It isn’t just about the Syrian lives subsequently lost but the powerful sense in the Commons that night that somehow nobody had won; that whatever one’s view of the decision, British democracy had stumbled to it for the wrong reasons. Even some of Ed Miliband’s team seemed oddly spooked by their victory. To them, this was a vote on one specific, hastily announced and badly explained US plan to bomb Syria imminently, and for obvious reasons they balked at being bounced once again into supporting American military action, no questions asked. They didn’t necessarily see it as the final word on intervention in Syria, yet that is what it became. We cannot afford such confusion this time.

• Gaby Hinsliff is a Guardian columnist and former political editor of the Observer