Garvan Walshe is a former National and International Security Policy Adviser to the Conservative Party. He runs Brexit Analytics.

Emmanuel Macron’s victory hasn’t so much changed the game of European politics as made its rules obvious. It has exposed three British illusions – that Britain can divide France from Germany; that there exists a group of “northern” countries who would prefer Anglo-German continental leadership to that of common European institutions; and that the project was doomed to fail – and will, in time, expose three more: that Europe’s financial services industry can only be centred on London; that they need us more than we need them, and that Britain can remain an important world power outside the EU.

Number Ten’s negotiating strategy – let’s not kid ourselves that in this Louis XIV-style administration any other centre of government gets a look in – trades on these illusions, and if it doesn’t change radically, it will fail. The worst form of failure would be leaving the EU without any agreement. Planes wouldn’t be able to fly, or money be transferred, to and from the continent. It would spark a financial crisis of Latin American proportions. Yet the swivel-eyed loons still say they want it (if only as a bluff they desperately hope won’t be called). A thumping victory on June 8th will allow the Prime Minister to face them down.

The second-worst form of failure would be to leave with only the barest elements of an agreement. It would avoid tariffs on goods, but impose major delays at the border. Services trade – by far the most valuable and fastest growing – would be left out. Robust estimates calculate this would reduce our total trade by between a third and a quarter, because distance still matters: as it doubles trade halves. There would no law-enforcement cooperation, or participation in common research programmes.

Yet, the EU’s negotiating position is that it would accept this minimal agreement only if the UK agreed to have the rights of EU citizens living here underwritten by the European Court of Justice. Their beef is with our doctrine of parliamentary sovereignty. They, Hungary and possibly Poland excepted, all have systems that prevent a simple parliamentary majority removing fundamental constitutional rights. We no longer do. Since Tony Blair became Prime Minister, our system has become so presidential, and our executive so strong, that Lord Hailsham’s fears that we have become an elective dictatorship are close to being realised. The only check on power of any substance is the press. And our European ex-partners don’t trust that we’ll keep their citizens’ rights safe when the Daily Mail brands judges “enemies of the people” or calls for “saboteurs” to be “crushed.”

On this no democratic government can afford to compromise. And the Europeans come into these negotiations in confident mood. With Macron’s victory they have now sent populists packing in Austria, Holland and France. Germany’s AfD, far from threatening Merkel’s hold on power, has consolidated it. The most ideologically pro-European man since Giscard d’Estaing now occupies the Elysée.

An alternative scenario could have been conceived: one where Austria elected a far-right president, Geert Wilders dominated the Dutch political scene, Marine Le Pen came to power in France, and Merkel faced a difficult campaign under pressure from the Bavarian right winger Horst Seehofer from within and the AfD from without. In such circumstances, Britain could have presented itself as a responsible polity that had addressed people’s concerns about immigration, while still maintaining its constitutional traditions. London would have found itself in a position to propose the looser European arrangement based on cooperation between national capitals it first advanced in the 1950s, in preference to the single continental legal framework, that was actually set up.

But this gamble didn’t pay off. It turns out that British worries about sovereignty and immigration from the continent are felt more intensely here than in the rest of Europe. They make leaving the EU a political necessity. Since the rest of the continent doesn’t want to accommodate this, it’s the British Government’s job to find a way out.

But if it is to do so with minimal damage – Brexit being necessary doesn’t stop it being a grave mistake – it needs to recognise the uncomfortable reality of Britain’s weak position. Their economy is five times the size of ours, their population seven and a half times. They held the populists at bay, whereas we embraced them. They pulled together to save Europe when we bet on escaping the “corpse” to which we were shackled. That image, beloved of Leave campaigners during the referendum, was first struck by British anti-Europeans immediately after the war. They were wrong then, and they’re wrong now.

The EU goes into these negotiations with renewed confidence. Its growth rate now exceeds America’s. A new and dynamic president of France, facing difficulties less serious than those Margaret Thatcher overcame, openly embraces its ideal. Nationalist populism very much on the back foot. And they at last have a common enemy against whom to unite: us.

Not since Henry VIII struggled to rid himself of his “Flanders mare” has London trucked with the continent from a position of such weakness. The new government would be well advised to change strategy. If the Prime Minister uses her election victory to plot a pragmatic and conciliatory course, she will find that her eminently pragmatic party will back her.