Theresa May has assured the nation that she has every confidence in Boris Johnson. Someone has to, I guess. He, in turn, must be hoping this backing does not resemble the notorious “chairman’s vote of confidence” so feared by football managers, invariably coming as it does so soon before the sack.

It has been a spectacular few days for the foreign secretary, even by his unrivalled standards. I fear I played a small part in his interesting weekend when I described him as “Trump’s poodle” for pulling out of his Moscow visit, reportedly on the “advice” of America. I also questioned from the get-go why he would set as pretty much his only goal at the G7 summit the delivery of sanctions on Russia.

Now threatening sanctions against Vladimir Putin is, on balance, necessary. The fear is that prodding the wounded bear will only make it strike out more savagely. But the use of chemical weapons against civilians crossed a moral (and legal) line, and Moscow needs to be clear on what side of that line it stands. Bashar al-Assad can’t be allowed to think he can get away with it.

But sanctions were, at best, a sideshow. The humanitarian imperative was – and still is – how to prevent the further massacre of the innocent. So the discussion needed to be about no-fly zones, safe corridors and gaining the cooperation of regional powers – all massive challenges, likely to take up the energies of the G7 summiteers.

Instead we had the Boris circus show: lots of table-thumping beforehand about how he was going to deliver sanctions (“We are the exact opposite of poodles,” his spin doctors briefed, a little hysterically), followed by that familiar quizzical expression at the post-summit photo call when he had delivered precisely nothing. Even fellow Conservative ministers have said, helpfully, that he has been humiliated, while No 10 has offered him every form of support short of actual help. May happened to be on holiday (no crime in that) but then saw her foreign secretary floundering without arm bands, out to sea and out of his depth. And instead of taking charge of the mounting crisis, she looked away.

But how could she possibly expect him to secure agreement with the G7, including as it does the major players in the EU? Johnson has, after all, dedicated his entire career to insulting them.

Just a few months ago he was threatening his hosts, the Italians, with a prosecco war. An Italian economics minister, Carlo Calenda, had explained to Johnson that Britain couldn’t enjoy all the benefits of the single market and immigration controls, so the foreign secretary told him Britain wouldn’t buy his prosecco. Calenda apparently replied: “OK, you’ll sell less fish and chips, but I’ll sell less prosecco to one country and you’ll sell less to 27 countries.” Calenda concluded: “Putting things on this level is a bit insulting.”

“Insulting” is one word for it, when Johnson is determined to rip us out of a single market at an estimated cost of £200bn over 15 years.

As for Germany, it was hardly amused when Johnson accused the EU of aspiring to Hitler-style hegemony. Such language might work for Nigel Farage, but for someone who would go on to be foreign secretary? Yet the Nazi comparisons slip swiftly from the Johnson tongue: he even refused to apologise after comparing the behaviour of the president of France to a second world war prison guard. François Holland’s crime? Defending France’s interests rather than rolling over to give Johnson the wildly unrealistic Brexit deal Johnson had dishonestly sold the British people. When his promises are exposed for what they are, Johnson simply turns to insult. When the EU27 have the trump cards on Brexit, it is hard to see how this negotiating tactic will secure Britain a good deal.

And this is what Conservative Brexit ministers gloating and briefing against Johnson should realise: just as Johnson was humiliated at the G7, so Britain will be humiliated in Brexit negotiations if ministers go in firing off demands like a hostage negotiation. You simply can’t have a good deal while demanding a hard Brexit, especially if you leave the decisions to Johnson rather than trusting the British people with a say on the final deal, as Liberal Democrats demand.

The Conservative Brexit government should be drawing the country together and strengthening friendships with key allies. Johnson, as we all predicted, has shown himself incapable, and must be sidelined. The real fault lies with May in leaving the court jester free to become the crown prince. Because Johnson’s antics shame – and weaken – us all.