In these bleakly unnerving geopolitical days, let’s try to lighten the mood for a few seconds with an antique club comic joke.

This family of nine is so poor that they sleep in the same bed, while all the parents can afford to give each of their seven small sons for Christmas is a tin helmet. That night, the dad staggers home drunk from the pub and crawls into bed. Every time he tosses and turns, the kids roll over and their helmets crash to the floor. Just as he finally seems to be dropping off, he perks up with, “Ah Bridget, you’re as lovely today as the day I married you.” “Hang on to your f****** helmets, lads,” mutters the six-year-old, “here we go again.”

In the Syria of sarin gas and Tomahawk missiles, the tin hat is an archaic redundancy. But so far as Britain’s role in an ever more incendiary Middle Eastern crisis, hang on tight all the same, lads and lasses, because here we go again.

Boris Johnson’s forced cancellation of his Moscow trip provides a merry ride down memory lane, back to the infamous Yo Summit at which Tony Blair begged Dubya to send him to the Middle East as peace envoy, and was casually rebuffed.

And the echo from that golden age goes beyond Boris Johnson’s embarrassment in being denied a Middle East-related diplomatic mission by Washington (no doubt with Downing Street’s enthusiastic consent). Britain is again offering the US uncritical support for a reflex, seemingly illegal military reaction based on patchy intelligence – however likely, it remains unverified that Assad was behind the nerve gas atrocity – with consequences that cannot be foreseen.

Boris Johnson: Syrian chemical attack evidence points to Assad

Whatever one thinks about the morality of Trump’s cruise missile assault, its impact on the behaviour of the Assad regime and the Kremlin is a matter of guesswork. What we do know, latterly from Libya, is that modern history teaches caution about emotionally sourced military interventions in the region.

But given the talent of British governments to avoid learning lessons, the welding of visceral disgust about the gas attack to the Brexit-heightened economic pressure to cling to America’s skirts made this diplomatic response depressingly inevitable.

What was less inevitable was the brutal humiliation visited on Boris by Theresa May. It was one thing for Boris to be stood down from his meeting with Sergey Lavrov, the silky Russian foreign minister, and not a pleasant one. No one wants to become the latest poster boy for British subservience to Washington, or to wake on the Sabbath to “Boris the ‘poodle’” (Moscow’s description) on the Mail on Sunday front page.

But you can understand Anglo-American unease about unleashing Boris on such a fraught and delicate diplomatic task.

With Lavrov expressly referring to how close the US came last week to engaging militarily with Russia, this may not be the moment to risk a notoriously loose cannon firing the starting gun for a Third World War with a misplaced: “Strike me pink, comrade, let’s have lashings of Grey Goose and liberate Crimea!”

Far more crushing to Boris’s ego than having to leave the big boy diplomacy to US Secretary of State Rex Tillerson is his usurpation at home. By sanctioning her Defence Secretary to hold the Kremlin responsible for the attack in the crudest and most aggressive terms (will the western powers never learn that, whatever our differences, Russia needs treating with respect?) while Boris was silent, May has effectively made Michael Fallon her de facto Foreign Secretary.

It isn’t always easy to feel sympathy for Boris. But seldom since Margaret Thatcher let it be known that making Geoffrey Howe her official deputy was a meaningless sop to his wounded ego has a PM humiliated a senior colleague so publicly.

Defence Secretary Michael Fallon says UK backs US airstrikes on Syria

How much more of this Boris’s pride can take may be the hour’s most urgent question. It wouldn’t break the top 20. Nonetheless, if May finds him too reckless for the job – or too independent-minded to do her bidding as her imperiousness demands – she should have the guts formally to replace him with the lavishly sycophantic Fallon, and take her chances with whatever vengeful mayhem Boris might rain on her from the backbenches.

Her mischievous treatment of Boris since giving him the punishment job last July (that “ffs” at PMQs, for instance, which Surrey’s answer to Sarah Millican mock-coyly revealed stood for “fine Foreign Secretary”) had already strayed into vindictiveness. Now it is closing in on brazen incompetence.

However significant Britain’s role in whatever unfolds in Syria and with Russia (every inch as crucial, you’d imagine, as Micronesia’s), it is amateurish to embark on it with a pair of demi-foreign secretaries – one muzzled in his kennel like a distempered labradoodle, the other her toy dog ventriloquist’s dummy.