In the coming days, perhaps as soon as Wednesday, Brexit will turn from abstract to concrete. A near-theological argument that raged in one form or another for nearly three decades will become hard and material, with a fixed deadline. Theresa May is about to trigger article 50, starting the clock on a two-year journey towards the exit from the European Union. And yet those in charge of this fateful, epochal process – and especially those who most loudly demanded it happen – seem utterly unprepared for it.

In four words, the European strategy for the Brexit talks has to be: pour décourager les autres

Philip Hammond’s budget on Wednesday illustrated the point neatly. The country is about to leave its largest export market, a decision with enormous economic implications. The chancellor had the floor for nearly an hour, his obligation to provide an assessment of the present and future prospects of the British economy. Did he so much as mention the imminent exit from the single market? No. Incredibly, he made just two fleeting references to the EU in the entire address.

Instead the stand-out measure, the one that has dominated political discussion since, was Hammond’s decision to take more tax from a core Tory constituency: the self-employed. Important for those individuals, most certainly; a political unforced error, no doubt. But for this to be the focus following a major economic statement on the eve of Brexit is displacement activity of the most heroic kind.

It’s as if the crew of the Titanic eyed the iceberg ahead and promptly decided to have a big squabble over whether to serve white or red.

This failure to wrestle with what’s coming goes wider. The public conversation since 23 June 2016 has barely differed from the debate before that date, each side – leave and remain – still refighting the EU referendum campaign, uncertain how to get out of the old groove.

That failing is most obvious among the Brexiteers, characterised by a refusal to own their victory and take responsibility for it. So when a voice of experience or authority dares point out the possible dangers ahead, they are either sacked, as was the fate of Michael Heseltine, attacked personally, like John Major, or else branded an “enemy of the people” who refuses to bow to the “popular will”.

Those with concerns are accused of “talking down the country” or lacking sufficient faith – as if, should Brexit make us poorer, the fault will belong to those who didn’t screw their eyes tight enough and believe. Credit to Jonn Elledge for calling this what it is: the Tinkerbell delusion.

This surely has to end with the triggering of article 50. From this moment on, the focus must be intensely practical. No more baggy rhetoric about sovereignty and “taking back control”. From now on, those who got us into this situation have to show they can get us out intact by March 2019.

That will require a major shift among the Brexiteer ministers and in Downing Street. Those close to the pre-negotiations between Britain and the remaining 27 EU states report an unwarranted hubris on the UK side that augurs ill. Too many Brexiteers cling to the campaign’s wishful thinking that we go into these talks as the stronger party, that “they need us more than we need them”, and that so long as we hang tough, the Europeans will buckle and hand us a dream deal.

Such arrogance is likely to be exposed soon. For one thing, it ignores the key structural fact that makes Britain’s negotiating prospects bleak from the start: namely, it is imperative for the EU’s own survival that the UK be left in a visibly, materially worse situation after leaving the EU than it enjoyed before. The logic is not vindictive. If the EU is to hold together it must prevent a Brexit contagion. Any divorce settlement must be ugly enough to ensure the remaining 27 stay with their spouse, no matter how loveless that marriage might feel. In four words, the European strategy for the Brexit talks has to be: pour décourager les autres.

But if British politicians are insufficiently mindful of that built-in obstacle, they are far too blithe about the sheer complexity of the undertaking that is about to begin. They are aiming to unpick 40 years of arrangements, seeking to annul them in a pact that will require the blessing of 27 other sovereign states.

To call it 27-dimensional chess understates the geometry: the final divorce settlement will have to be ratified by 38 different national and regional parliaments. To say nothing of the European parliament, commission and council. Each of these bodies has its own interests, pressures and red lines.

May will have to craft a document that satisfies every one of those competing forces, as well as both chambers of the UK parliament. She will have to do it without pushing Scotland towards a second, more winnable independence referendum or recreating a hard border between Northern Ireland and the Irish republic. And she has to get it done in roughly 18 months. Not for nothing did Dominic Cummings, the mastermind of the Vote Leave campaign, tweet with a candour rare among Brexiteers that leaving the EU was the “hardest job since beating Nazis”.

Or reflect on the supposed aces Britain is confidently looking forward to playing in the upcoming game of Brexit poker. Charles Grant, the sage director of the Centre for European Reform who predicted the leave vote, patiently explains how each one of these assets – which Brexiteers believe will make the Europeans putty in our hands – could create as much angst as advantage.

It’s true, says Grant, that the City of London is valued for the financial services it provides to the EU. But it’s also true that Paris, Madrid, Milan, Frankfurt, Dublin and others are circling, ready to feast on the City’s carcass: they want some of that business for themselves.

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The Brexiteers reckon the Europeans won’t want to give up London’s special relationship with Washington. But, says Grant, British “fawning” over Donald Trump alienates many Europeans, making them doubt we share their basic values. As for Britain’s contribution to European security – via its UN seat, Nato and its fabled military – that’s much admired. But not if it’s used as a threat: give us a free trade deal or we’ll pull out the 1,000 British troops recently deployed in the Polish-Baltic area. Talk like that will backfire.

Leavers should be approaching this gargantuan task with a special humility, because it was they who needlessly inflicted it upon us.

Remainers need to adjust to the new reality too. Many may be hoping that, as the price and consequences of exit become ever clearer through these talks, some among the 52% will gradually switch sides. But remainers should contemplate the less cheery prospect that the most ardent Brexiteers, and especially the anti-EU newspapers, will double down in their loathing of Brussels. When the EU 27 demand, say, serious cash for single market access, the Mail and Sun will dip their pen into an even deeper well of venom.

So remainers will need to handle these next two years carefully, readying themselves for the day when the deal is done, and ensuring they have already placed two key questions in the front of the public mind: is this deal better than the set-up we had on 22 June 2016? And if it isn’t, why are we doing it?