Boris Johnson wanted to be prime minister long before he wanted to take Britain out of the EU. The novelty of Brexit among his ambitions prompts a thought experiment: imagine there was no referendum. By some other craft Johnson has reached Downing Street. Would he propose swapping full EU membership for the latest version of a deal being hammered out in Brussels today?

Of course not. It would be a bizarre surrender of international status and economic leverage in the world’s largest trading bloc. He would see no wisdom in quitting the single market, erecting new borders and retreating from the rooms where big continental decisions are made. Even the fiercest Eurosceptics did not advocate that path before 2016. They thought there must be a way to leave the club without relinquishing members’ privileges. Seeing Brussels as a parasite on sovereignty, they did not understand that EU membership was a component of British power.

That is the cardinal error of Brexit, but its significance is easily lost in the detail. For days there has been an intense focus on customs arrangements. Could something be devised to allow Northern Ireland to trade as if it were still in the EU, while reassuring hardline unionists that they have left? Apparently so.

But it is important to step back and recall why this problem existed in the first place. Johnson envisages a future in which the UK does not shadow EU market rules. The greater the regulatory divergence on either side of a border, the harder that border has to be. Zoom out still further and you see a statement of intent by the UK to gain a competitive edge by undercutting its former EU partners. The original economic conception of Brexit is to make Britain a low-cost lure for investment. Brussels doesn’t like the sound of that. The more aggressively Britain intends to price itself relative to the rest of Europe, the harder the EU will make it for British goods and services to be sold within the single market. (What Johnson isn’t telling British voters is that the competitive edge he promises will be achieved by internal devaluation – shaving down wages, in real terms, and working conditions.)

Johnson’s wrangles in Brussels look like a re-enactment of Theresa May’s negotiations, but pull the lens back and you see a vital difference: May was persuaded of the need for strategic proximity to the rest of Europe and saw economic alignment as a necessary condition to achieve that. Her only intervention in the referendum campaign was a speech arguing that British national interests had to be represented in EU institutions and could be amplified there. She never rejected that view but tried unsuccessfully to find a Brexit model that could accommodate it.

Johnson’s view is different. He does not feel the tension between economic divergence and strategic alliance. He doesn’t see beyond his immediate desire in any endeavour, whether in private self-indulgence or public policy. As one former cabinet minister puts it: “He’s no chess player. He thinks one move at a time.” In Johnson’s world, the UK can develop an economic model conceived by people who despise the EU, alongside a foreign policy in which UK-EU relations are unaffected. His rise to power has been fuelled by ideological anti-Europeanism. He is the anointed candidate of Tory hardliners who hoped that Brexit would cause the whole European project to unravel.

Yet he also seems to think that EU leaders do not notice or do not mind. The prime minister has built a career on crass anti-Brussels caricatures and now imagines he can dissipate ill will by inserting jovial references to “our European friends” in his speeches. He is mistaken. Westminster might have lost sight of strategic dilemmas generated by Brexit, but EU leaders have not. Initial hope that the UK might still be accommodated within the broader scope of a European project has given way to anxiety about the emergence of a volatile lone-wolf state on the continent’s doorstep.

It is understood well enough in Brussels that Brexit and Trumpism are ideological siblings

Anti-Brexit arguments dwell on the UK’s welterweight status in trade negotiations compared to a 27-state bloc, but Britain is still a pretty powerful country: a G7 economy with a permanent seat on the UN security council and, by European standards, a big army. EU membership has never stopped Britain pursuing its own foreign and security policy – as Tony Blair’s clashes with Jacques Chirac over the Iraq war demonstrated. But generally, UK power has been plugged into the European project to the benefit of both. What happens when that plug is pulled?

Diplomatic protocol on both sides of the Channel is to insist that trade decoupling will not damage other types of partnership. Privately there is grim recognition that the world doesn’t work like that, not any more, not with Donald Trump in the White House, hurling tariffs across the Atlantic as part of a weird, multi-pronged attack on former western allies.

It is understood well enough in Brussels that Brexit and Trumpism are ideological siblings. They are fuelled by the same nationalist rhetoric, cheered by the same enemies of European solidarity and abetted to varying degrees by the Kremlin. Nor has it escaped attention in Paris and Berlin that Johnson tried to shut down parliament illegally, suggesting a cavalier approach to good governance. There are no reserves of personal trust in the prime minister and he has hitched himself to a doctrine of cut-throat competition that, whether he understands it or not, sets the UK on an economic collision course with Brussels. Even with a deal, the consequences will be messy.

Brexit was conceived and supported by people who want the EU to fail, yet any UK government that is committed to the rule of law and democracy should want the EU to thrive. That is the irreducible dilemma Britain faces. Those have been the options since 2016, obscured by technical jargon, shrouded in dishonest rhetoric, bedevilled with detail. One way emboldens forces of division and nationalism; the other way strengthens the project that was founded to resist those forces. Johnson chose his path and the stupid tragedy of it all is that he doesn’t understand where it leads but is dragging the country there anyway.

• Rafael Behr is a Guardian columnist