What will happen if Jeremy Corbyn wins? Will it be a nightmare on Downing Street, as Liam Halligan suggests in this week's Spectator? Or might Corbyn not be as bad as his critics fear? Helpfully, France provides a useful parallel of what prime minister Jeremy Corbyn might mean for Britain. And it doesn't make happy reading for the Labour leader.

It's Spring 1981 and France, the fifth largest economy in the world, elects the most left-wing administration since before the Second World War following eight years of conservative rule. The government immediately begins implementing its radical manifesto: nationalisation of 11 industrial conglomerates and most private banks, higher tax-rates at the upper levels and a special wealth tax. There is also a 15 per cent rise in the minimum wage and a 25 per cent boost for social benefits. A 39-hour working week is introduced. So, too, are five weeks' paid holidays as standard and retirement at 60, increased old age pensions, greater workers’ rights, free prescription charges and regularisation of illegal immigrants.

And, of course, massive government borrowing. The country’s new socialist leader lays flowers on the tomb of a great socialist hero. Culture minister Jack Lang proclaims the country to have ‘moved from the shadows into light.’

Fast forward to Spring 1983. Two years of spiralling budget deficits, uncontrolled national debt, three currency devaluations, falling exports and a dramatic increase in unemployment have left the economy in a parlous state.

Bowing to the European communities’ demands for financial rigour, the country’s socialist leader implements an austerity policy. Economic difficulties and cuts provoke cabinet divisions, key ministerial resignations and widespread anger at the left’s betrayal of its electorate. The reshuffled socialist government suffers badly in the 1983 local and 1984 European elections. That socialist government is finally defeated in the 1986 general elections when a conservative prime minister is elected.

Ignoring Karl Marx’s cliché that history repeats itself – first as tragedy, then as farce – British voters should recall the lessons of François Mitterrand’s France of 1981-6 when they head to the polls next week. The administration that took power was the most left-wing – with four communist ministers – since the 1936 Popular Front.

A mixture of blind idealism, government inexperience and desperation for power meant it promised too much too quickly. The left had not been in power since the inception of the 5th Republic in 1958. And Mitterrand, the leader of the socialist party, was determined to rectify that.

So what can this failed socialist experiment tell us about Jeremy Corbyn and what he would or wouldn't do to Britain? Mitterrand's experience is not merely an object lesson in naïve idealism and governmental inexperience. It also lays bare darker human failings of demagoguery, hypocrisy and mendacity inherent in the scramble for, and accession to, government.

Before coming to power, Mitterrand railed against the 5th Republic’s constitution. He dismissed it as a republican monarchy that granted Gaullist presidents the right to dispense arbitrary state power.

He said so forcefully in his 1964 best-seller Le Coup d’État Permanent. In opposition he had supported socialist manifestos proposing to dispense with two aspects of that presidential power: France’s nuclear weapon and the French intelligence services. Yet by 1985, when he was in power, he tasked France’s intelligence services with blowing up the Greenpeace ship Rainbow Warrior in Auckland Harbour. At the time, the boat was preparing to demonstrate against French nuclear testing in the southern Pacific. A journalist was killed in the blast.

Mitterrand also went against his previous ideals when, between 1982 and 1986, he created an additional intelligence cell at the Elysée, ostensibly to combat terrorism. It was not long before this unit was soon spying on writers, lawyers and political personalities likely to tarnish the president’s image by revealing, for instance, the existence of his illegitimate daughter. Only after his death did a Paris court judge Mitterrand to have been its ‘inspirer and essential decision-maker’ of this key unit.

Even now, France is paying the price for Mitterrand's time in office. Could Britain still be doing the same in 2050 if Corbyn becomes PM next week?

France's example certainly suggests so. The country has some of the highest personal and company taxation in the OECD, restrictive work practices, high labour costs and structurally high unemployment.

Attempts to roll back the 1981-83 reforms have proved difficult – for leaders on both the left and right – as this week’s massive strikes against president Macron’s reforms demonstrate.

In Macron’s view, the price of France’s ossified economy is systemic mass unemployment. For a decade before 1981, France’s unemployment rate had never touched six per cent. By 1985, it hit 9.3 per cent. France had moved into a permanent state of mass unemployment that has never regressed below seven per cent. Even today, in an era of full employment in many large western economies, it stands at 8.6 per cent.

The long-term effects on France’s socialist party are stark. Mitterrand limped on as president, forced to endure cohabitation with conservative prime ministers doing the dirty austerity work. Though he was re-elected in 1988, it was on a very different manifesto to 1981; even then much of his policy was constrained by more conservative cohabitation governments up to 1995.

Not until 2012 would another socialist president, François Hollande, be elected on a moderate platform. He too was remarkably unpopular and managed to serve only one five-year mandate. Since then, the socialist party has been all but wiped off the French political scene, polling a mere 6.2 per cent in the 2017 presidential election.

Britain may not be France. But the Mitterrand experience is, or should be, a sobering French lesson for a potential Corbyn government and those thinking of voting for it.

Those who will not learn even the plainest lessons from history—that of others as well as their own—may find themselves condemned to repeat it. Unfortunately, they would be forcing everyone else to repeat it too.

John Keiger is a former professor of international history in the Department of Politics and International Studies, University of Cambridge