“Never interrupt the enemy when he is making a mistake,” tweeted Labour’s international trade spokesman, Barry Gardiner, last week, quoting Napoleon. Like most of Labour’s frontbench he had opted not to engage with Theresa May’s post-Salzburg strop, preferring to let the Tories tear themselves apart unmolested.

But Napoleon also said that when the enemy is in disarray you should send in the cavalry pronto. With the Tories flailing, it is time for Labour to unleash a two-pronged political offensive: it should give a clear commitment to a Norway-style deal, and promise a second referendum to ratify it if achieved.

Though it edged towards this in a composite motion agreed on Sunday night, the position is still frustratingly unclear.

Labour’s refusal to cement its desired end state for Brexit was defensible until July. Jeremy Corbyn’s team wants, above all, to bring down Theresa May’s chaotic administration and get an early shot at government. If it achieves that, launching a radical project of growth and social justice, it could change the political atmosphere across the developed world.

But Labour’s leadership has struggled to understand that, since the Chequers deal fell apart in July, the Brexit issue has become the primary weapon to achieve a Labour government. This is partly due to the memory of 2017. In the last election, Labour activists found that as long as they assured people Brexit would happen, the issue evaporated on the doorstep, allowing them to talk about healthcare, local services and employment rights.

But if, as has been suggested, May’s advisers are planning a second snap election, this would unleash – yet again – a “back me or sack me” campaign focused around Brexit. With the looming possibility of food and medicine shortages, and a deranged tabloid press revelling in the prospect of no deal, it would be hard for Labour to distract voters from the central issue a second time.

In addition, the EU’s flat refusal to discuss the Chequers proposal signals to Labour, just as curtly as it signalled to May, that there are only two options: a Norway-style deal that leaves Britain inside the single market, and a Canada-style free trade deal. Under intense pressure from members dissatisfied with the current line, Labour delegates have agreed to table a resolution committing Labour to “full participation in the single market”. That is significant.

Last December, the Institute for Public Policy Research presented Labour with a white paper advocating Swiss-style “alignment” with the single market, rather than participation. As with May’s Chequers plan, the IPPR wanted Britain to align voluntarily with some single market rules, but diverge from others in order to promote growth and social justice. Though the shadow cabinet refused to endorse the IPPR plan, the document is the closest Labour has ever come to drawing up its own version of Chequers.

But the Salzburg summit showed that a voluntary alignment option is no longer possible. May has wasted two years, and expended almost all the UK’s negotiating capital, only to get her version of it rejected. Labour’s new conference motion, committing the party to participation in the single market, appears to recognise that.

Corbyn’s strategy to date – of spelling out red lines and opposing solutions that crossed them – was always designed to allow Brexit-supporting Labour voters to learn by experience how treacherous the process would be. The Tories had told them that leaving would be easy; it wasn’t. They told them the new treaties would be done and dusted by March 2019. They will not be. They assured voters there would be no downsides to a no-deal solution. Now they urge voters to stockpile food in case it happens.

The gameplan worked up to a point. May’s chaotic negotiating stance has eroded popular support for Brexit, though not decisively. But the process of learning by experience now has to be supplemented with a positive vision, reinforced by Corbyn and Keir Starmer spelling out a clear Brexit proposal of their own.

Theresa May during the EU summit in Salzburg. Photograph: Sean Gallup/Getty Images

My position on Brexit has always been a compromise between principle and what’s needed to assemble a voting coalition to put a leftwing Labour government in power. If it had been possible to achieve a leftwing Brexit, I would have supported it: the Lisbon treaty is a machine for destroying jobs and growth, and is stoking disillusion in democracy. But the real-world forces pushing for Brexit in 2016 were always those of xenophobic nationalism, and their project was an even more deregulated and polluted Britain. So I campaigned on remain and reform.

Once we lost, the challenge was how to craft a bespoke deal that respects the result, allowing working-class communities to move on from the bitter arguments that had dominated their public spaces. But after Donald Trump’s election victory, the options for such a deal narrowed.

If Brexit was a crack in the superstructure of globalisation, Trump has aimed a wrecking ball at its foundations. As the world fragments into competing trade and finance blocs, it is imperative for both geographic and cultural reasons for Britain to attach itself as closely to Europe as possible.

With Trump in power, being inside the single market has become the only logical option for a Labour government, even if that might make some of its plans for state ownership, state aid and workplace regulation more difficult to achieve.

I refused to back calls for a second referendum so long as May was engaged in meaningful negotiations to enact what both the referendum and parliament had voted for.

But after Chequers collapsed, and the Tories had to rely on Labour Brexiteers to defeat the customs union proposal, it became obvious that there could be no majority in this parliament for any of the government’s desired Brexit outcomes. The best option is a general election – the sooner the better. In that election, for Labour, the promise of a second referendum could be a powerful weapon – especially if it was combined with a clear outline of a Norway-style deal.

The shock of Salzburg is likely to push the Tories towards a hard, Canada-style free trade agreement. In that case, the way is open for Labour to make a new, broad offer to British civil society and business. A Norway-style deal, and a second referendum to ratify it and confer democratic legitimacy, could create an electoral coalition ranging from fervent remainers to Brexit voters who just “want it over with”, and would resonate strongly with businesses desperate to remain close to Europe.

For hard remainers that too would be a compromise between principle and reality. But if progressives in the political class want to live on this island amicably with the 17 million people who voted Brexit, we have to deliver what they voted for, in the least damaging way, and then offer the entire electorate a final chance to say whether they like the outcome.

A Norway-style deal would also be a lifeline to manufacturing and agriculture. It would leave Britain as a “rule taker” – but so do all other outcomes, such is the global influence of Europe as a regulatory superpower. And it would leave the path clearer for those who want to rejoin the EU, should public opinion change.

For clarity, if a Norway-style deal could be agreed, with assurances from Brussels that Labour’s programme of state ownership and labour market reform would be permitted, I would vote for it in a second referendum. I don’t want to see working-class communities divided for another decade over this issue, and policymakers deflected from the urgent global challenges of automation and climate change.

So Labour’s new composite on Brexit is welcome, but it’s not enough. In a fully democratic party it would be open to amendment – but don’t hold your breath. Corbyn needs to use this moment of intense Tory disarray to get on the front foot. While it’s great to watch the Tories make mistakes, you don’t win battles without attacking.

Because, as Napoleon also said, “Imagination rules the world.” People go into political battle for ideals, not checklists: Labour needs to move on from red lines and objections, and spell out a positive vision for Britain as part of the European single market.

• Paul Mason is a writer and broadcaster on economics and social justice