Until quite recently, many people fervently believed this day would never arrive. Just over three months ago, Brexit finally seemed to have pushed British politics to the brink of a nervous breakdown. There were tears and tantrums in parliament. There were petitions and protests outside. A litany of lawsuits was waiting in the wings to derail the whole process. Ardent remainers felt they were tantalisingly close to stopping Brexit altogether. The future looked wide open.

From a second referendum to a government of national unity, from toppling over the cliff edge to beating a full retreat, almost anything appeared possible. Yet here we are – one day after formally leaving, just as the British people decided on 23 June 2016 – and all that is gone. What happened? Were the last four years a mirage?

The emotions were real. It was the sense of possibility that was an illusion. In recent years, some surprising political outcomes have seemed to pass from being impossible to being inevitable without ever taking the time to be merely improbable. Jeremy Corbyn’s election as Labour leader was like that: what couldn’t conceivably happen suddenly couldn’t be stopped. Brexit has taken us on a much more circuitous journey.

Before the referendum, plenty of people assumed that leaving the European Union was out of the question. Then, in the immediate aftermath of the vote, it looked like a fait accompli. Shocking though the result might have been, the choice was clear and we would all have to live with it – just like any other election result.

Yet over the following months and then years, Brexit drifted back into the realms of the improbable. There were so many potential barriers in its way, and so many people determined to raise them, that what had seemed like a done deal gradually became an open question. Opposition, which might have been expected to dissipate over time, instead grew. Rather than a result to which the losing side needed to reconcile itself, it became a historic injustice to be avenged.

It was as though the Brexit referendum had torn a hole in the normal functioning of British politics. We had passed through a portal to somewhere strange – and the job was to find a way to return to the place we had left behind. The opponents of Brexit took heart from the fact that because the referendum had apparently upended the old political order – along with their own sense of order – it ought to be possible to upend Brexit. The way to undo the referendum was to treat it as a rupture in the political space-time continuum, and to repair the tear.

This was a mistake. The Brexit referendum did not stand outside the normal run of British politics. It was a product of that politics. The illusion was to believe that direct democracy, in the form of a high-stakes referendum, had somehow derailed parliamentary democracy and that therefore the task was to get it back on track. But Brexit all made sense – and only made sense – as a product of parliamentary democracy. An elected government, enacting its will through parliament, made the referendum happen. Only an elected government, enacting its will through parliament, could deal with the consequences. The old rules still applied. Everything else was just for show.

The show included the growing demands for a second referendum. It was a passion play, full of its own heroic performances. This demand made total sense for anyone who believed that the first referendum was an outlandish event that needed to meet its match: having passed through the portal we required some equivalent way of passing back. But if you started from the premise that we had never left politics as usual, then pushing for a second referendum was always a quixotic enterprise.

A People’s Vote march in London, October 2019. Photograph: Wiktor Szymanowicz/Barcroft Media

There was no way to get there. Parliament would have had to agree not only that it was a good idea, but also on the question to be asked, the rules under which it was to be held and the meaning of its outcome. This was obviously impossible in the existing parliament. But any parliament that could not agree on Brexit would not have been able to agree on a second referendum. And any parliament that can agree on Brexit doesn’t need one.

Why did the illusion take such a strong hold over so many people? As with much else in contemporary politics, there was a generational divide. Among older remainers, for whom the result of the referendum was wildly outside the bounds of what they understood as political normality, it seemed like it had to be countered in kind, with something equally abnormal. For younger remainers, especially those in their late teens or early 20s, the last few years had been the only politics they really knew. Wasn’t this – the chaos, the emotion, the sense of possibility – how it worked? No. But it’s easy to see why it would look like that if that’s all you’ve encountered.

The general election of 2017 helped to perpetuate the illusion. Labour came very close to another enormous shock. It looked once again like anything might be possible. But Labour only managed to do as well as it did by parking the issue of Brexit. Both main parties went into that election promising to respect the result of the referendum. It was only when Theresa May’s failure to win a majority produced a weak and vulnerable government that it seemed as though the result might have been a rebuke to Brexit itself. It wasn’t.

In 2017, when the fate of Brexit was not on the table, people voted in a way to make Brexit harder to achieve. But in 2019, when two years of parliamentary stalemate had put Brexit in serious doubt, they voted to make Brexit happen. The seeming possibility of stopping Brexit was an unintended byproduct of the voters having assumed it was bound to happen. Once that assumption came into question, Brexit became inevitable again.

It was not only on the remain side that it sometimes felt as though everything was up for grabs. Plenty of Brexiteers were deceived, too. Within the parliamentary Conservative party the despair was palpable during the nadir of May’s government. The fear was real that not only might Brexit be lost, but the party itself might be wiped out. In last spring’s European elections the Conservatives polled the lowest national vote share in their history, while Nigel Farage’s Brexit party was riding high. But the Tories have one great advantage at times like these. When no one knows what to do, Conservatives focus on their parliamentary survival. And since parliamentary survival is the name of the game in a parliamentary democracy – nothing about the Brexit referendum had changed that – it often works.

The person who saw this most clearly was Dominic Cummings. At a low point in the fortunes of Boris Johnson’s new government last autumn, when it had been defeated in the courts, was being stymied in parliament and mocked in the press – “Classic Dom!” – Cummings gave a rare public appearance in which he said that compared to winning the referendum itself, what he was going through as the prime minister’s chief adviser was “a walk in the park”. It sounded shockingly arrogant. He can’t have meant he knew he was going to win in the end – no one ever knows that. But it did mean that he knew the only way to win. He had no choice but to focus on the fact that the existing parliament would never agree on how to stop Brexit, and to get to a new parliament his opponents would have to give him a general election under circumstances that did not suit them. The rest was just noise.

Cummings was convinced that the really hard part had been to win the referendum itself, given that the old rules still applied. He had needed to shock the existing system. Engineering a successful public vote to take Britain out of the EU in the face of establishment opposition – including in parliament, where most MPs were strongly opposed to the idea – was truly an outrageous achievement. But since the old rules still applied, turning the referendum result into reality through parliament was far more straightforward.

All he had to do was reabsorb the shock back into the existing system. The status quo would take care of the rest. The iconoclast could safely reinvent himself, if only for a few months, as an upholder of the established order. Even the failed prorogation of parliament, which probably generated the most passion of all, was another distraction. Cummings wasn’t really trying to get round parliament. He was just trying to get parliament to function like it used to.

With hindsight, we can see that the illusion Brexit might be stopped made it easier for Brexit to happen in its current, harder form. Opportunities were missed to moderate it. If Labour MPs who were tempted to vote for May’s deal early last year had actually done so, we would have had a softer Brexit than the one that is now looming. But hindsight creates its own illusions. May’s deal became softer as she became weaker in parliament; her weakness in parliament made it much harder for Labour MPs to support her deal. Those who considered it were accused of “complicity with Brexit”.

Dominic Cummings ‘wasn’t really trying to get round parliament. He was just trying to get parliament to function like it used to’. Photograph: Hannah McKay/Reuters

Propping up a wounded prime minister under those circumstances looked like treachery. For all the sound and fury, few politicians felt they had much of a choice. They were boxed in by the growing intensity of sentiment on all sides, as much as they were by parliamentary arithmetic.

From the day of the referendum result, Brexit was probably inevitable. It sounds fatalistic. But it doesn’t have to be: if the old rules still apply, then from today everything changes again. Up until this moment, the Brexit vote was something that parliamentary politics needed to acknowledge – it had to be swallowed, somehow. But now it’s been swallowed, it ceases to be an event that any government is required to accommodate. It becomes something that this government owns.

It also ceases to be the people’s responsibility. It’s now Johnson’s responsibility. This means the will of the people can no longer be so readily invoked in the face of parliamentary opposition. The people didn’t make his deal and the people didn’t pass this legislation. The people aren’t going to negotiate the future relationship either. It’s on Johnson and his ministers and his chief adviser. So the opposition is free to let rip.

It’s true that the people chose this government and this prime minister at the last general election. So isn’t it still ultimately the voters’ responsibility? But that’s not how parliamentary politics works. The voters don’t own the consequences of their choices at election time. They reserve the right to blame the politicians they pick for everything that subsequently happens. We don’t choose the policies in a representative democracy. We choose the politicians. And if anything goes wrong, it’s the politicians we will get to pillory.

As it passes from unspecified future event to messy present reality, Brexit has gone from being something that almost nothing could derail to something that almost anything could. Coronavirus? A global recession? A trade war between the US and China? None of these is about Brexit, but any of them could make Brexit a much more precarious business. If that happens, we will know who to blame. It will be Boris Johnson’s fault, even though none of that is really his fault – unlike the referendum result, which really was his fault, and for which he could never be made to shoulder the blame, no matter how hard people tried.

If we want an idea of how things might go from here, one possible parallel is with the Iraq war. Of course, in that case there was no referendum beforehand. But there were huge street protests, passionate opposition, massively divided public opinion and a widespread sense of looming calamity. It couldn’t be stopped, because the opposition could not muster enough support in the one place that mattered: parliament. Both of the two main parties were full of doubters, but it proved impossible to make those doubts decisive in the face of a prime minister with a large parliamentary majority determined to forge ahead. The Liberal Democrats, who became the vehicle for parliamentary opposition, never had the numbers to make the difference.

The voting public, who had elected the Tony Blair government that did the deed, somewhat reluctantly re-elected that government in 2005. But the voters never blamed themselves for what happened. They blamed their rulers for having lied to them. They took their time, but when they got the chance the people took their revenge, in part by ejecting Labour and helping the Liberal Democrats into coalition government. In some ways, the people have been taking their revenge ever since.

One of the long-term consequences of the Iraq war and the mistrust it sowed in the British political system was Brexit. Revenge in such cases can be a dish served very cold indeed. The long-term consequences of Brexit – the Brexit that this government is now undertaking on the people’s behalf but no longer in their name – are likely to be equally profound. Some governments will eventually be broken by it, perhaps even including the current one.

Once the people had voted for it in the referendum, Brexit was bound to happen in some form. But now that it is happening in its present form, the people are free to think again. The last four years created the impression that British politics had entered a new space of possibility. That was an illusion. The real possibilities start today.

• David Runciman is professor of politics at Cambridge University