Jeremy Corbyn's rise to the Labour leadership heralds an era of ideological contest that threatens Britain's membership of the EU - and the United Kingdom itself.
When does cosy consensus become groupthink? According to the social psychologist, Irving Janis, it is when the desire for conformity becomes so strong that alternative courses of action are not even considered, let alone taken.
Jeremy Corbyn, the uncompromising left-winger who has never held ministerial office, surfed from Labour's backwaters to party leader on a wave of groupthink. The British left never fully accepted Blair's Third Way - and his greatest mistake, the Iraq war, provided the pretext for its demonisation of him. Corbynistas disparage the party's centrists as "red Tories" - a process Janis defined as 'stereotyping' opponents as spiteful and biased. The British left has always seen itself as the guardian of political morality, leading to a state of total certainty in which the risks flowing from the group's decisions - withdrawal from NATO might endanger the country's security, for example - are reflexively dismissed. And Janis also pointed out that moral certainty encourages excessive optimism: the British left imagines that the surge of Corbyn backers signing up to vote will be replicated in the broader electorate, despite the fact that no leader from Labour's left has ever won a general election.
The stable liberalism of the pre-2008 period is crumbling, giving way to ideological contest between three political tribes - a Corbynite left, the Conservatives and the Scottish nationalists - which imperils Britain's membership of the EU, and the future of the UK.
Between 1992 and 2008 there was consensus over the big policy questions of the age: that the state should reflect and nurture the country's social liberalism, and provide more rights and opportunities for minorities and women; that it should intervene in markets only to correct obvious failures; that pro-work redistribution through tax credits and a minimum wage should counter poverty and inequality; and that more should be spent on improving public services. Now, Britain's parties are retreating into ideological comfort zones, ignoring or attacking evidence that contradicts their prior beliefs, and choosing policies less on a careful analysis of outcomes than on tribal orthodoxies.
Corbyn's policies engage in a debate with a spectral Margaret Thatcher: re-open the coal mines that she closed; subordinate monetary to fiscal policy; unpick the privatisation programme that she started. He has no programme of progressive structural reforms - to property, land and retail finance markets, or to the tax system - which would be efficient ways of reducing Britain's troublingly high level of inequality and raise its weak level of productivity. Confronting past enemies, the left does not notice the alternative roads it might travel.
Labour is not alone. The Conservative policy programme follows party ideology over a careful analysis of the country's problems. The EU referendum will define Cameron's second term, even though Britons still rank Europe below immigration, the economy, health, welfare and housing in importance. They are right to do so: it is hard to find a major problem facing the country that would be solved by leaving the EU. Meanwhile, George Osborne's decision to move towards a budget surplus, reached predominantly by cuts to government consumption and capital spending, is ideological small-statism: higher public investment in infrastructure and housing is needed to cope with a growing population, and investment finance is currently cheap as interest rates are low. The Conservatives' proposed laws to make industrial action harder are simply a political trap for Labour, since Britain's days lost to strikes are half the EU average, and are hardly a major drag on the economy.
As for the third force in British politics, the Scottish National Party's rise lies in the fact that Scots have come to define their political identity against the Tories. The SNP has a tendency to make eye-catching policies to maximise the contrast with those south of the border, irrespective of whether the policies work. Abolishing university tuition fees was a canny political move, since they are hated by the predominantly middle-class students that pay them, and fees in England have risen to £9,000 a year. But abolishing fees led the SNP to cut bursaries for poorer students, to the extent that they are worse off under the new system. The SNP's decision to make medical prescriptions free costs 7.5 per cent of the Scottish health budget - money that could be better spent on hospital and social care, given that Scotland's population is ageing rapidly.
Britain's tribal warfare threatens to undermine the country's political and economic settlement. Jeremy Corbyn is at best equivocal about Britain's membership of the EU: he sees it as an agent of international capitalism. Under pressure from Labour moderates, many of whom said they would quit the shadow cabinet if he did not support EU membership, Corbyn has said that Labour will campaign to remain in the EU. But he also said he would try to reverse any "damaging" reforms David Cameron negotiates, citing an opt-out from EU employment rules as an example - and criticised the EU's proposed trade deal with the US, the Transatlantic Trade and Investment Partnership.
But the threat Corbyn's leadership poses to the UK's EU membership arises mostly in the internal dynamics of the Conservative party. The Tory right, knowing that Labour has little hope of winning the 2020 general election under such a left-wing leader, has less incentive to maintain party discipline. Eurosceptics will be tempted to follow their gut feeling, since a split over Europe will not be hugely damaging with the electorate.
The pro-EU coalition on the centre ground of British politics is shrinking, as is the opinion poll lead for the In camp. And since the EU's member-states face intractable problems - refugees and the euro's flaws - which aggravate British euroscepticism, the referendum could hardly be held in worse circumstances.
As for Scotland, Corbyn's supporters say a left-wing Labour party will draw voters lost to the SNP back to the fold, and make Scottish independence less likely. There are two reasons why this is doubtful. First, there are not enough left-wing voters in England and Wales for Labour to win the 106 seats needed for a majority in 2020. Scots will have little faith that Corbyn will deliver them from Toryism. Second, Nicola Sturgeon, the SNP leader, has popularity ratings in Scotland that are matched only by Angela Merkel in Germany.
The SNP has drawn up a list of 'triggers' that it says should prompt a second independence referendum. Some of the mooted triggers are unlikely to work - a renewal of Trident, Britain's nuclear deterrent, because a majority of Scots support a deterrent; or if the Conservatives take Britain into an illegal war, because illegality is difficult to prove. But if the majority of Brits vote to leave the EU, with a Scottish majority voting to stay in, Sturgeon would justifiably argue that the constitutional settlement that Scots approved in the first independence referendum was no longer in place. And Scotland would probably vote to leave in a second vote.
Britain's move towards the international margins and its inability to confront underlying social and economic problems are the result of its widening political fault-lines. If the country does break up, its failure to rise above groupthink will be to blame.
John Springford is a senior research fellow at the Centre for European Reform.