Not even the Boris-loving Brexit press could rouse much warmth this morning for his damp squib of a speech. Even the Telegraph, Boris Johnson’s campaign HQ, was struggling – but it did its best: its editorial ends with a call for the prime minister to “send him out to sing for his supper more often”. Well, bring it on. Vacuity, vanity and frivolity, decked out with decrepit jokes but devoid of fact: more of this stuff will discredit him further with every outing.

The country, Europe and – according to the Japanese ambassador leaving Downing Street in outspoken despair last week – much of the world, is waiting with growing alarm for this government to reveal what it means to do. Brexit problems escalate daily, but the foreign secretary only added to perplexity and irritation.

Dublin, Holland, Belgium and France are already preparing their ports for the tariffs and customs checks that look only too likely, following Johnson’s Downing Street-sanctioned speech announcing the hardest of Brexits. In Dover and all the other UK ports, nothing has been done, too late now to avoid two-day queues and 20-mile back-ups.

Play Video 2:57 Boris Johnson: Brexit is not 'a V-sign from the cliffs of Dover' - video highlights

But the outstanding shocker was failure to mention Ireland on the day the Stormont talks broke down again. Is that surprising? Johnson may flaunt a little historical knowledge, but the history of Britain and Ireland is of no interest to him and his ilk. How far back should you go? Since Henry II landed in Waterford to declare himself Lord of Ireland? The Irish on both sides of the border have long memories, not least of the potato blight famine of the 1840s, which killed a million, caused another million to emigrate, and was ignored by Britain. Johnson’s negligent attitude mirrors his party’s age-old contempt. Theresa May tipping up for just one day announcing she could solve the DUP/Sinn Féin standoff was another display of casual Conservative arrogance.

Yet she and her government have signed up to phase one of the withdrawal from the EU deal with a pledge of “full alignment” in all important regulations between north and south, ensuring no border friction. That is entirely incompatible with everything Johnson said yesterday. He is – unbelievably – our foreign secretary and Ireland is our closest foreign neighbour, yet he plainly thinks it can simply be batted away as an annoying irrelevance. It can’t be.

History shows how every British government that ignores “the Irish question” ends up paying the price. The EU 27 are standing by Ireland, and will agree no bespoke deals or special treatment that does not ensure that “full alignment” and a soft border. Like it or not, Ireland holds the key to our Brexit future. Ignoring that essential fact shows a foreign secretary entirely unfit for his job.

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His true purpose, we all know, was to secure the votes of Tory MPs to ensure his name is on the two-person ballot paper whenever the prime minister departs, and to win over his small Brexit-fanatic party membership in the shires.

But his claimed purpose was to reach out to unite this mortally divided nation. The man who did most to break the country in two could, possibly, have been the man to try to heal those wounds – if he were serious, politically adept and cleverer than he is.

But peace and compromise are not in his nature. What he brought was not an olive branch for the 48% but the hardest of Brexits, escaping from all the burdensome EU regulations we are “lashed” to. Naturally none were spelled out, since naming them would reveal precisely what his wing of his party has in mind – and that would look mighty brutal, from working rights to the environment and food and product safety.

But suppose instead he had acknowledged why the closeness of the referendum result obliges his party to offer the softest of Brexits to bind up the national rift, he could have proved himself a worthy contender. Instead, with his weary old tropes, he missed his moment to surprise.

A Boris Johnson premiership may happen, God help us, but he showed yet again why he is supremely unfit for any high office. Yesterday he could have been Henry V, casting off the Brexit misdeeds of his youth: instead he reaches for the crown as the unreformed vain wastrel he always was, eager as ever to put self before country.

• Polly Toynbee is a Guardian columnist