Theresa May’s expression is hard to read at the best of times, and almost impenetrable at the worst. So it proved on Sunday when she made her second appearance in less than a month on the BBC’s Andrew Marr Show. Was the embattled prime minister boldly channelling Marshal Ferdinand Foch during the first Battle of the Marne in 1914: Mon centre cède, ma droite recule, situation excellente, j’attaque (“My centre is giving way, my right is retreating, situation excellent, I am attacking”)? Or did her delphic mask recall the terror one imagines she felt in her youth when wickedly “running through fields of wheat”?

If May is apprehensive, she is absolutely right to be. Last week was bad enough – two senior cabinet resignations, Donald Trump’s helpful interventions – but this week the legislative substance of Brexit returns to the floor of Commons, in the form of the taxation (cross-border trade) bill and the quite distinct trade bill.

This video has been removed. This could be because it launched early, our rights have expired, there was a legal issue, or for another reason. Theresa May says Trump told her to 'sue EU' – video

Amendments aplenty have been put down by Brexiteer MPs, partly as a means of testing the numerical strength of opposition to the Chequers agreement. On Wednesday, what remains of that deal – torn Post-its and doodled images of Michel Barnier – will be the subject of general debate or a “neutral motion”. The next day, David Davis’s successor as Brexit secretary, Dominic Raab, will take questions in the House.

The detail of all these exchanges may be mind-numbingly technical – you just try to break the ice at parties with a one-liner about VAT and the EU – but they symbolise something elemental, deeply unsettling, awesome in scale. This is not just another parliamentary process. It is the drawing up of a new blueprint for Britain’s role in the world, being carried out in the most fractious and blinkered manner imaginable.

For May, there are two interrelated perils. First, the Labour remainers cannot be relied upon to support the Chequers deal and the white paper it spawned as the best hope of a soft-ish Brexit. This was the significance of Peter Mandelson’s article in the Observer, savaging the blueprint as a pathway to “national humiliation”.

The Chequers Brexit compromise offers the worst of both worlds | Peter Mandelson Read more

At some point, May will lose a significant vote on Brexit in the Commons in such a way that Jeremy Corbyn will find himself under intense pressure to initiate a motion of no confidence in the government. Were the Labour leader the statesman-in-waiting he claims to be, rather than merely the head of Europe’s biggest political club, May would already be long gone. His hand may soon be forced.

Second, the Tory party might soon decide that May has outlived her function as the half-effective glue of an otherwise-collapsing government: the first Pritt Stick to hold the title of privy counsellor she may be, but all Pritt Sticks stop working eventually.

I do not trust claims that Graham Brady, the chairman of the 1922 Committee, has already received close to the 48 letters required to trigger a confidence motion in the party’s leader. What is certainly true is that the remainder of May’s authority – fragile at best since last year’s election – is ebbing away fast and has not been restored by her cabinet reshuffle last week.

Boris Johnson, David Davis, Jacob Rees-Mogg and others are all on manoeuvres, and the first two have resignation speeches to make. As the late Margaret Thatcher discovered in 1990 when Sir Geoffrey Howe had his say, such speeches can make or break a premiership. So vulnerable is this balsa-wood government that a single blast of well-aimed rhetorical aggression could trigger its demise.

Yet the dangers in all this are not confined to the prime minister. The personal dilemma facing May is just a pathetic parable for the much deeper problems facing her party and, indeed, the nation. What the Chequers summit and its grim aftermath have demonstrated is the true nature of Brexit, or rather the sheer nightmare of translating an essentially emotional decision into a practical commercial, institutional and diplomatic arrangement. The hard Brexiteers simply have no patience with the complexities of the disentanglement that they seek: whether those complexities concern the Irish border and the Good Friday agreement; or the need for a “common rulebook” for food and goods; or the white paper’s recognition that the new immigration rules will have to “support businesses to provide services and to move their talented people”.

Theresa May’s grand plan has left her stranded in no woman’s land | Andrew Rawnsley Read more

The Brexiteers hate detail – but, like Trump, they love anger, recrimination and the language of treachery. In the Sunday Express, Rees-Mogg declared that No 10 has behaved in a way that “a more severe commentator would call … untrustworthy”. Steve Baker, who resigned as a Brexit minister last week, identifies a “cloak and dagger” plot masterminded by an “establishment elite”.

It was Daniel Finkelstein, now a Tory peer and Times columnist, who observed when working for William Hague that the more bloody-minded Eurosceptics “would not take yes for an answer”. This has proved to be terribly true. The Brexiteers refuse to act as though Britain is engaged in a negotiation with 27 other nations, preferring to aim at alleged traitors at home than to consider how best to bring about the departure from the EU that they so desire.

Profile Who is Steve Bannon? Show Hide Born in Norfolk, Virginia in 1953, Steve Bannon was the chief executive officer of Donald Trump’s election campaign in its final months in 2016. He later served as the president’s chief strategist for seven months during the early phase of his administration. He was fired in the summer of 2017, but Trump is recently said to have been talking about him positively. The bluntly spoken, combative Bannon was the voice of a nationalistic, outsider conservatism, and he pushed Trump to follow through on some of his most contentious campaign promises, including his travel ban on several majority-Muslim countries.

He led the rightwing Breitbart News before being tapped to head Trump's campaign, where he pushed a scorched earth strategy.

After Trump fired him, Bannon launched a European operation called the Movement. Based in Brussels, it was set up to give far-right parties access to polling data, analytics, advice on social media campaigns and help selecting candidates. “Remember ‘Bannon’s theorem’,” he told the Guardian at the time. “You put a reasonable face on rightwing populism, you get elected.”

Bannon, who served in the navy and worked as an investment banker at Goldman Sachs before becoming a Hollywood producer, had been hosting a pro-Trump podcast called "War Room" that began during the president's impeachment proceedings and had continued during the pandemic. He was arrested in August 2020 and charged with fraud over a fundraising campaign called We Build the Wall. Photograph: Carlo Allegri/X90181

All of which connects to another, transnational campaign. As gripping as Trump’s visit to these shores was, the simultaneous presence of Steve Bannon, his former chief strategist, was possibly more significant. Bannon’s focus, I understand, is next year’s European elections – his aim being to mobilise a nationalist caucus of MEPs in the European parliament. He also maintains strong contacts in the UK and – like Trump – has concluded that Johnson is the right successor to May.

Never underestimate the populist right, especially when being assisted by US and Russian sympathisers. Its principal protagonists have curated the “Brexit betrayal myth”: the claim that the British volk has been let down by a craven elite of multiculturalists and theatre-goers. Some of their number argue that Ukip should be revived under Nigel Farage. But the more dangerous plan is to colonise the battered Tory movement in the years to come – like a facehugger from Alien – flooding local associations with like-minded members, and turning the party of Disraeli, Macmillan and Churchill into a Trumpite nationalist force.

These are possibilities, not certainties. But the volatility that has created them is sufficient cause for deep concern – and not only to May. As profound a disappointment as she has been, she is only a symptom of a much deeper crisis. It is a historic crisis of political, cultural and national trajectory, in which, to be frank, the identity of the prime minister is only a second-order question.

• Matthew d’Ancona is a Guardian columnist