David Davis has become the sixth cabinet minister to resign from Theresa May’s enfeebled government in eight months. He is, however, the first to do so over Brexit.

For more than a year, as May’s EU policy incrementally softened, no Brexiteer left the cabinet. But Davis, who had been regarded as the likeliest to resign (having threatened a comical number of times to do so), has finally walked. The Brexit Secretary regarded the Chequers agreement – which backed remaining in the single market for goods – as an unacceptable breach of May's promises. Davis had also been irretrievably sidelined as Olly Robbins, May’s chief Europe aide, took control of the process.

No.10 had prepared for resignations in advance of the cabinet summit and will seek to retain authority by swiftly replacing Davis. May is also mindful of a possible confidence vote (triggered by 48 Tory MPs privately requesting one), which allies and foes alike believe she would win. But the Prime Minister's position is still in greater danger than at any time since her electoral humiliation.

Yet regardless of who is in the cabinet or, indeed, who occupies No.10, the realities of Brexit are unchanged. Only a plan which involves a customs union and remaining in the single market for goods can prevent the creation of a hard Irish border (which the UK has repeatedly pledged to avoid) – and parliament will reject any deal that does not. More importantly, so will the EU.

In July 2016, Davis wrote that “within two years, before the negotiation with the EU is likely to be complete, and therefore before anything material has changed, we can negotiate a free trade area massively larger than the EU, trade deals with the US and China alone will give us a trade area almost twice the size of the EU” That prediction, to put it mildly, hasn't aged well.

As some Leavers are now grasping, the great irony of Brexit is that it has left the UK more subservient to Brussels than ever. When the EU drew up the divorce proceedings it did so with the intention of maximising control. No member state, it assumed, would be so reckless as to invoke Article 50.

The Brexiteers contend that the government’s failure to adequately prepare for “no deal” has weakened its hand. But even were this true, the debate is now entirely academic: there are just months left to reach an agreement – and failing to do so would be an immense act of economic self-harm. The absence of cabinet resignations last week was correctly interpreted as proof that, to adapt Margaret Thatcher, the Brexiteers have no alternative.

But Davis, who resigned as an MP 10 years ago to trigger a by-election over 42-day pre-trial detention, has left all the same. After the conclusion of the Chequers meeting, May insisted that her cabinet – more than two years after the EU referendum – was finally united. That facade lasted just 48 hours.

For May, Davis’s resignation at least suggests that collective responsibility may reign once more. Rather than pissing inside the tent (as Boris Johnson continually has), Davis has left it. But the Brexit Secretary, who some Tories believe retains leadership ambitions, could yet lead a revolt from the backbenches.

Never before has the government’s weakness been more dramatised. Having triggered Article 50 recklessly early in March 2017, May is now left with painfully little time to secure an agreement. For the UK, the most dangerous instalment of the Tories’ Thirty Years' War has begun.