If we hadn’t fully grasped it during the period of phony war that began on 24 June last year, now we know: for the foreseeable future, Brexit and its fallout will define British politics, and subjects that were once barely discussed – trade, tariffs, the fate of our farms and fisheries – will be at the heart of the debate. But so too will the most profound questions our politicians have to deal with: what role Britain can now expect to play in the world, what kind of society will emerge from the mess, and whether the increasingly fragile United Kingdom can hold together.

This is, then, arguably the most momentous period the country has entered since 1945 – which brings us to the Labour party, and its quite astounding irrelevance.

The basics of its position barely need recounting. Labour has no hope of forming the next government, nor in all likelihood the one after that. Questions about its possible extinction abound. And its staggering poll deficits leave the Tories to do pretty much as they please. A YouGov poll this week found that – and read this slowly – more people who voted Labour in 2015 would choose May as prime minister than Jeremy Corbyn: across the population as a whole, the idea of the latter as the best option for PM is supported by a miserable 13%.

More people who voted Labour in 2015 would choose Theresa May as prime minister than Jeremy Corbyn

Having endlessly bumbled around the central questions of Brexit, Labour has now come up with six “tests” for the government’s eventual agreement. They may be built around an impossible insistence (even if it is a quote from Brexit secretary David Davis, the chances of any deal containing the “exact same benefits” the UK currently gets from the single market and customs union are nil), but they at least represent a hardening of the party’s position. The problem is that in the hands of the leadership, they seemingly cannot cohere into any easily understood stance. On Wednesday’s BBC Brexit special, this was Corbyn’s problem: an awkward mixture of woolly abstraction and pernickety detail, and the familiar air of a man filling airtime, because that’s what he has to do, not someone who has anything much to bring to the moment.

Meanwhile, in the absence of meaningful leadership, Labour is doing what it always does when its compass breaks, and collapsing into arcane squabbling. Thanks to talk of an imminent alliance between the Corbyn-supporting group Momentum and the Unite union, Tom Watson is at daggers drawn with his former flatmate, Len McCluskey. There is a low hum of chatter about how party rules may or may not change to ensure that candidates from the hard left can easily get on to any future leadership ballot, which is all part of the war between the party’s supposed left and right, and the sense that a small number of Labour faction-fighters are having a whale of a time.

Facebook Twitter Pinterest In Germany, ‘Martin Schulz has managed to turn round the fortunes of Labour’s supposedly doomed sister party, the SPD, to the point that he now looks like a possible contender for power’. Photograph: Steffi Loos/Getty Images

If this had anything to do with the wider political situation, it might be excused. But no: these are turf wars, largely relevant only to the people involved. After a testy meeting of the parliamentary Labour party 10 days ago, one unnamed MP told journalists that the party’s condition is “worse than 1985”. Depending on your tastes, that verdict might either bring to mind Marx’s adage about history being repeated first as tragedy then farce, or the immortal words of Jay Gatsby: “Can’t repeat the past? Why of course you can!”

Self-evidently, this pantomime is a very British version of the crisis that is besetting social democratic parties across Europe. The party’s current malaise is partly down to how tightly the Brexit moment is bound up with a new kind of politics – centred on identity, belonging and nationhood – which causes people on the left a paralysing kind of discomfort. In an even wider context, it is pretty clear that some of the left’s most treasured beliefs – in the efficacy of the centralised state, and the glories of lifelong, secure employment – are all but obsolete. If Labour is floundering, there are deep reasons for that.

There again, look at Germany, where Martin Schulz has managed to turn round the fortunes of Labour’s supposedly doomed sister party, the SPD, to the point that he now looks like a possible contender for power. I am no fan of the school of politics that would have you believe great leaders can miraculously laugh in the face of deep historical currents; even if Schulz has quickly surpassed many expectations, he and his party do not exude a confident sense of the future. But clearly there is a difference between blithely hurtling into catastrophe and trying to lead a party away from it.

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What is perhaps most baffling is this. A big part of what happened last summer has now hardened into political cliche: in what used to be considered Labour’s core areas, a lot of people voted to leave the EU because of the chance to alert the political establishment to problems that had for too long been ignored. For a brief spell after the referendum, all this – which often seemed as much moral as economic – was the “in” thing to talk about: communities with little sense of collective purpose; a worsening housing crisis; the UK’s awful regional imbalances; an economy that offers too many people mere scraps.

Now, in the hands of politicians confident enough to use all that in the service of a national story, all that stuff should surely be brought back to the heart of the debate. The need for forensic scrutiny of the government’s negotiations speaks for itself. But if Brexit is indeed some moment of national rebirth, what kind of country are we going to create?

Seizing that opportunity would require the kind of urgency, clarity and energy that Corbyn and his allies simply do not possess. To state the blindingly obvious, if he was as loyal a servant of his party and movement as he often says, he would be on his way. This may turn out to be Labour’s moment of eclipse. For the rest of the party, the question screams out for an answer. What are they going to do about it?