Reactions to the Chilcot Report suggest that the only remaining argument for the Iraq War is that criticism rests on hindsight. Tony Blair explained that he invaded the country 'in good faith', as if that somehow excuses the catastrophe that followed. General Sir Mike Jackson shrugged off responsibility for British soldiers blown up in inadequately armoured vehicles by observing that their fate was unforeseeable: ‘we'll never know,’ he said, if any died unnecessarily. Listening to the deflections of blame, I was reminded of a scholar whose work Tony Blair has long admired. Philip Bobbitt warned in 2004 that the war's critics were making a basic philosophical error. He called it ‘Parmenides' Fallacy’: the mistaken attempt to assess a situation 'by measuring it against the past, as opposed to comparing it to other possible present states of affairs’.

The argument sounded clever when I first read it, but not for long. It was clear already that the invasion threatened unplanned chaos (with Islamic State barely a glint in a few jihadis’ eyes), and when photos of the Abu Ghraib atrocities emerged six weeks later, the recklessness became clearer still. The problem with Bobbitt’s dictum is that it can retrospectively justify any course of action. No matter how badly things turn out, there's always a counterfactual hypothesis that suggests they could be worse. All it takes to prolong a disaster is political rhetoric – as the compounding catastrophe in Iraq vividly proved.

The fallacy, which really ought to bear Bobbitt’s own name rather than Parmenides’, has now lost whatever credibility it once had. Blair still insists that invading Iraq made the world safer, but few of the politicians, media proprietors and ordinary citizens who once backed him contend today that keeping the peace in 2003 would have been more cataclysmic than war. The EU referendum makes the link between foresight and hindsight worth reconsidering, all the same. Having embarked on another high-stakes adventure, the United Kingdom is gambling again – but this time, most practicalities lie ahead, and nightmare scenarios have not yet come to pass.

There's no disputing that 51.9 per cent of voters supported Brexit, but the implications of the vote are anyone's guess. It's unclear if the withdrawal notice prescribed by Article 50 of the Lisbon Treaty has to be initiated by ministers or Parliament; legal challengers are arguing for legislators to have their say. The Department of Brexit promised by Theresa May is hardly even embryonic. The UK currently has about twenty civil servants capable of renegotiating its trade agreements with the world, 700 fewer than necessary. Extracting 43 years of EU law and jurisprudence from the body politic will also subject the civil service and all four UK legislatures to unprecedented legal puzzles. Activities governed directly by EU rules will have to be covered by new domestic legislation, and thousands of statutes and regulations are going to require amendment – unless MPs grant ministers draconian powers to rewrite them. Existing EU-based rights and obligations won't simply evaporate either, and courtrooms are sure to be haunted by their zombie presence for years to come.

Progress will demand impressive leadership, especially because many Leavers thought they were voting to sweep aside technicalities, and it’s far from certain that Theresa May has what it takes. She’s pragmatic, but that’s no guide to future policy choices, and her belated acquiescence to the referendum result is out of tune with the 51.9 per cent majority. The Leavers were united only by the mysticism of Brexitology – prophecies of a prosperous sovereign kingdom, confidently back in control – and May is at her persuasive best only when she foresees difficulty and hardship on the long road ahead.

And that is why the Bobbitt Fallacy deserves reconsideration today. One of the few undeniable facts about the Iraq War is that its most eager advocates lacked foresight. Millions psychologically compensated for that by reimagining their pasts: 54 per cent of British people supported military intervention in 2003, but only 37 per cent recalled holding that view twelve years later. That doesn't establish that Brexiters will reconsider, but it's a reminder that wishful thinking is always fluid. Turning away from the EU may very easily become the UK’s most deeply regretted decision since the Iraq War, and if it does, every expectation disappointed and every promise broken by May’s government is going to have political reverberations.

The referendum result can't be erased. Just as the consequences of reckless adventurism aren't automatic, however, simple acquiescence isn't the right response. To an extent that didn't happen after Iraq's invasion, party allegiances look set to realign, while the vote's ramifications are going to be contested for years. And as the national mood shifts, the government will be duty-bound to take note. The people have spoken – but in a democracy, a bare majority's mistake should never shackle a majority that has changed its mind.