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

In less than a month the tanks had streamed up the banks of the Tigris and Euphrates to encircle Baghdad and defeat Saddam’s once fearsome Republican Guard. Political prisoners had been released, and the remnants of the Ba’ath Party had fled to their traditional strongholds near Samarra and Tikrit.

To Brexiters, the first days of the Tory conference in Birmingham must have felt like those weeks in spring 2003 felt to me. There were warning signs: the US Army, untrained in non-lethal policing, shot peaceful demonstrators in Fallujah. The crowd that gathered to pull down the statue of Saddam wasn’t particularly large. The looting in Baghdad, perhaps inevitable, went on too long. But a horrific genocidal dictator had been deposed, Iraqis could finally say what they thought, its Shias could take their rightful place in the country’s governance, and Kurds begin to chart their own destiny. The war’s opponents’ bloodcurdling predictions of house-to-house fighting in Baghdad, tens of thousands of coalition casualties, and even Saddam Hussein using chemical weapons (that those self-same opponents also claimed did not exist) against his own people or Israel had not materialised. Their “project fear” stood decisively refuted. Once the Americans and British, practiced state builders in Kosovo, Sierra Leone and Bosnia, put their reconstruction plans in place, Iraq whole and free would get to its feet again.

How we cheered Tony Blair when he told a packed house of Commons “people said this would be my Vietnam.”

That those people turned out to be right was due to five serious mistakes in strategy. More than tactical errors (such as disbanding the Iraqi army, or waiting too long to impose order) or bringing the wrong tools to the job (trying to battle an insurgency with army designed for high-intensity fighting) they sprung from fundamental flaws in reasoning about what the Iraq mission entailed.

This government’s Brexit strategy suffers from exactly the same flaws.

1. Dismissing Inconvenient Experts

As the Iraq war was being prepared, specialists warned that sectarian identities suppressed by Saddam would reemerge; that regional powers would interfere; that regime loyalists would not go quietly; and that Islamist extremists would exploit any power vacuum to regroup. Instead of developing plans to address these problems the war’s supporters denied their existence.

The same is happening over Brexit. The financial markets’ confidence in the management of the British economy has been shaken, and major foreign investors are withholding judgement because of this refusal to acknowledge the magnitude of the readjustment in economic relationships a Brexit that takes us out of the single market would require. It is not adequate to suppose that countries outside Europe can replace disrupted European trade. To point to the relatively faster growth in trade with emerging economies is to mistake a quantity for the rate at which it changes. America excepted, our trade with non-European countries is not very large, and so is growing from a low base. The EU accounts for two fifths because it is close and rich and companies have established deep commercial relationships across the bloc. Some of these can eventually be rebuilt elsewhere but it will take time for businesses to grow those new relationships and for the UK to negotiate the international trade frameworks they require.

If the Government believes that the people have instructed it to prioritise attaining greater sovereignty over maintaining existing economic relationships it needs to prepare for the costs to the Exchequer of this adjustment that its own civil service has estimated as being in the tens of billion of pounds. It needs to think about which budgets are to be cut and by how much; how much extra government borrowing will be needed over the next decades to cover the shortfall and whether there is scope for further borrowing for a fiscal policy stimulus. These costs will increase if the markets question Britain’s economic competence: attacking treasury civil servants or questioning the independence of the Bank of England only makes the cost of compensating for the economic dislocation of Brexit even greater.

Nor is it enough to point to the rising FTSE to head off the falling pound. The FTSE is only higher when measured in sterling. Measuring a building’s height in feet instead of metres doesn’t make it any taller. Though a weak currency makes it easier for people to come to terms with falling living standards because of what economists call the money illusion, that doesn’t stop living standards from falling. The benefit to exporters is limited because in today’s economy firms import so many of their components, while prices of food and fuel will rise for everyone.

2. Believing your own propaganda

We had told ourselves that Iraqis would not only be grateful for Saddam’s removal, but also happy to work under our tutelage to establish a pro-Western democratic regime there. This, our own propaganda, turned out to be completely wrong.

The equivalent Brexit mistake is to overestimate the ease with which Britain can do side-deals to obtain goods and services market access while closing its labour market to Europeans. Government ministers still talk about Germany’s desire to sell us their cars and France’s to supply us their wine and cheese. They need to get real. The reason England is uncomfortable with the EU is that it is a political project not just an economic one. That is why its continued existence means a lot more to the Germans and the French than the size of their exports of stereotypical goods.

We have to accept that in these negotiations we will have far less influence than we would like. The Brexiteer claim that “we sell more to them than they sell to us” is wide of the mark because it’s the size of trade relative to the economic bloc in question that matters, and their trade with us is a smaller part of their total economy than our trade with them is.

Together these facts mean that a hard Brexit at bearable economic cost has advantages for them over a complicated deal that creates special arrangements for Britain but which would cause political trouble for the EU itself. If we enter negotiations thinking of hard Brexit as an opening offer from which we can be bargained down, we could end up finding they accept an offer we had intended for them to refuse.

3. Mistaking the reasons used to justify the mission with what’s required to make it work

Weapons of Mass Destruction, thought to have been possessed by Saddam, were the central part of the case for regime change in Iraq. Though perhaps the best (if ultimately not good enough) hope for getting the UN to approve the invasion, the extent to which the case relied on them undermined support for the war once they couldn’t be found, and diverted attention and resources to finding the supposed WMD instead building a peaceful and orderly Iraq after the war.

Immigration was the referendum’s WMD, without which Leave wouldn’t have won, and the main immigration lie — that it is economically bad for the country — was enthusiastically endorsed by the Remain campaign itself(!) which agreed that immigration was too high but wanted people to vote for an outcome that wouldn’t bring it down.

Now the Government has decided that the vote to leave is, to the exclusion of everything else, a vote to reduce immigration. In this they underestimate the intelligence of the public. A majority of the people do want to bring immigration down, but, as this ComRes poll shows, not if it costs them money. Unlike the Government, for whom it has become an absolute position, and the basis for a clumsy attack on an international financial elite more suited to Ken Livingstone than a leader of the Conservative party, the people understand there are trade-offs to be made.

So instead of a serious discussion about what immigration regime is right, sensible and compatible with which trade and market access arrangements, the Government has denied itself being able control immigration policy while offering a policy that would benefit Poland, Spain, Italy and a host of smaller EU member states, supply much needed skilled and unskilled labour and tax revenue to a British economy at essentially full employment.

4. Forgetting that our reputation among foreigners matters to foreign policy

The invading coalition ignored the wider strategic consequences of the invasion and the way it was conducted. Britain’s and America’s reputations suffered in the region and across Europe; and the reaction to Iraq meant we were too reluctant to intervene in Syria, allowing Assad to get away with war crimes as bad as Saddam Hussein’s.

In the same vein, if you are about to undertake difficult negotiations with foreign countries that will determine your country’s future for decades, it’s not such a good idea to describe their citizens living here as “bargaining chips,” and promise to free our health service of the scourge of foreign doctors. Nor was it sensible to propose to collect a database of foreign workers and tell the media its purpose is to shame companies that employ them. It’s more than a few decades since such ideas would have found favour in Berlin.

5. Shock and awe doesn’t get you very far

A successful reconstruction of Iraq needed its communities to cooperate and its neighbours at least to acquiesce; but an incompetent and brutal occupation alienated the former, and left a power vacuum whichthe latter filled.

Brexiteers and the newspapers that support them have taken to wielding the “will of the people” with the enthusiasm of French revolutionaries. David Davis even took to describing the 52-48 result, which can reasonably only be said to be clear, as “overwhelming.” But if the British Election Study is right, enough leave voters have already changed their minds to flip the result. Their numbers will only grow as Brexit’s economic damage is felt, and protest voters turn against the government of the day. Unless efforts are made to win them back to the Brexit side the Government will be stuck implementing Brexit against public opinion instead of with it.

We need a Brexit Surge

As things stand, my gut instinct is that the Government could still manage to get a hard Brexit through, if it threatened an election against a hopeless Labour party and managed to manoeuvre its way around the Fixed Term Parliaments Act— not least because the EU could well find it straightforward enough to agree. But I’m not the sort of Leninist remainer who thinks Brexit should be tested to destruction so the UK can slink back in sans rebate, sans opt-outs, sans everything.

Brexit is the most serious undertaking a British government needed to do since the Second World War and the Government needs to rise to the challenge the voters have given it. This means it needs to be honest with itself and the voters about the difficulty of the task they have been set (no more refusing to listen to foreign academics or dismissing Treasury economists’ calculations as bogus); stop believing that side-deals to British advantage will be easy to obtain; treat immigration as the important public concern that it is, not the overriding one it isn’t; stop threatening the citizens of the countries it has to negotiate with; and start making serious efforts to bring the country, through its representatives in Parliament, the devolved administrations and mayors, with them. Call this the Brexit Surge.

This would require a radical change in style from a Prime Minister who prefers to operate by consulting only with the tightest of circles. But what served her well enough managing the Home Office will not be enough to allow her to make a success of the people’s instruction to Brexit. She should hope the courts accept Lord Pannick’s argument that Article 50 can only be invoked after an Act of Parliament and give her the chance to change policy without appearing to climb down.