In May 1981, a left-wing leader was elected on a programme of mass nationalisation, wealth taxes and a higher minimum wage. François Mitterrand was the first Socialist president of France’s Fifth Republic and promised a “complete rupture” with capitalism (his government included Communist ministers for the first time since 1947). Twelve industrial conglomerates and 38 banks were taken into public ownership, including Paribas and Banque Rothschild, and the minimum wage was increased by 10 per cent.

“We want to develop a mixed economy. We are not revolutionary Marxist-Leninists,” Mitterrand reassured the French employers’ federation. But his socialist experiment faced immediate resistance. Capital fled France at a rate of two billion francs a day and private firms launched an investment strike. Inflation, meanwhile, reached 12.6 per cent and the country’s trade deficit surged. The European Monetary System (EMS), a precursor to the euro, which pegged the franc to the deutschmark, limited the scope for currency devaluation and capital controls. “I am divided between two ambitions: constructing Europe, and social justice,” lamented Mitterrand.

Faced with a choice between leaving the EMS and pursuing economic protectionism, or remaining and accepting the dictates of the market, the president chose the latter path. Under the tournant de la rigueur, or “austerity turn”, of 1982-83, public spending was cut by 44 billion francs, taxes on workers and consumers were increased by 40 billion francs, budget deficits were capped at 3 per cent of GDP and wages were delinked from prices. The most ambitious postwar socialist experiment in western Europe was over.

Mitterrand’s fate remains resonant today. After the left-wing Greek party Syriza entered office, it faced a comparable choice between euro membership and austerity (embracing the second). For Jeremy Corbyn’s Labour, which has adopted its most radical programme for decades (including the renationalisation of public utilities, the abolition of university tuition fees and a £10 minimum wage), there are also lessons.

Socialists are wise to hope for the best but prepare for the worst. At a fringe event during the recent Labour conference, the shadow chancellor, John McDonnell, revealed that he was planning for the economic woes that afflicted Mitterrand. “What if there is a run on the pound? What happens if there is this concept of capital flight? I don’t think there will be, but you never know, so we’ve got to scenario plan for that.”

The UK’s current account deficit of 5.9 per cent of GDP – the largest of any major economy – reflects its dependence on what Mark Carney, the Bank of England governor, calls “the kindness of strangers”. If international investors lost confidence in Britain, the pound would further depreciate, increasing inflation and reducing living standards.

The French experience demonstrated the importance of internal unity. Mitterrand was restrained by his more moderate finance minister, Jacques Delors, who prioritised currency stability over Keynesian reflation. Corbyn, by contrast, defied the wishes of trade union leaders to name McDonnell, his closest political ally, shadow chancellor.

Both Syriza and the French Socialists faced external constraints: the EMS and the euro. The Labour left’s long-standing Euroscepticism reflects its desire to avoid such limitations. Though the restrictions imposed by the EU’s single market are smaller than some suggest, the UK would be even freer to pursue state intervention outside of it.

Labour’s 2017 manifesto was modest compared to Mitterrand’s 110 Propositions for France. But the implementation of a left-wing programme in an economy far more globalised than in the 1980s would pose a significant challenge.

“In economics, there are two solutions. Either you are a Leninist, or you won’t change anything,” Mitterrand eventually concluded to his aides. A Corbyn government would be the greatest test of that thesis since the Frenchman's botched experiment.