Almost six centuries after she was burnt by the English, President Nicolas Sarkozy will lead a commando operation tomorrow to free Joan of Arc from captivity.

Not from English captivity but from her status as a foreigner-bashing, official heroine of the French far right. Friday marks the 600 anniversary of Joan’s birth. Mr Sarkozy will take time out from rescuing the French and European economies to attend a series of events in her native village of Domrémy-la-Pucelle in the Vosges. He will also visit Vaucouleurs in Meuse, where Joan or Jeanne or Jehanne (1412-1431) spent the early part of her brief career as a religious visionary and resistance leader.

The xenophobic National Front adopted Jeanne d’Arc as an icon of ultra-nationalism two decades ago. The NF will celebrate her 600 birthday with an open-air rally led by the party’s leader Marine Le Pen and its founder, Jean-Marie Le Pen, in Paris on Saturday.

President Sarkozy first attempted to wrestle Joan from far-right ownership during his bid for election in 2007. So did his principal rival, Ségolène Royal. Although the president is not yet a declared candidate, his appearances tomorrow will, in effect, mark his entry to this spring’s presidential campaign.

Implicitly or explicitly, Mr Sarkozy will, like Joan, pose as the saviour of a French way of life menaced by arrogant external forces and internal divisions and weaknesses. He will imply that he, not the National Front, is the true guarantor of French nationhood.

There is nothing new in posthumous attempts to recruit Joan of Arc to political or religious causes. She was virtually forgotten by the French for 400 years, until her re-invention first as a patriotic-republican and then as a religious-conservative heroine in the mid 19th century. In the US and Canada, she has become a feminist icon: a symbol of girl power. In Latin America, she is claimed by the revolutionary Left as one of the first popular, resistance leaders, a female Che in chain mail. In 1920 Joan was canonised by Rome as part of a campaign by the French church to recapture the soul of France from godless republicanism.

But who was the real Jeanne d’Arc? The story is well known. A cross-dressing peasant girl who came from nowhere, directed by “voices” of saints, to lead the French armies and defeat the English. She lifted a cruel siege of Orléans; created a sense of French nationhood; changed the course of the Hundred Years War; but was betrayed by French traitors and burned by the English as a heretic and a witch.

The problem is that, according to recent French historical studies, little of that is true. Jeanne never really led the French armies. She was a kind of mascot in armour. She was repudiated by the French king and ended as an independent, guerrilla leader. Her entire military career lasted just over a year. The Hundred Years War continued for 22 years after her death. In any case, her enemies were also French and Burgundian in what amounted to a desultory, muddled, three-way civil conflict.

Joan was sent by the dauphin, later King Charles VII, to relieve Orléans in April 1429, as part of a food convoy. She galvanised the city's defenders, who were under no real threat from a relatively small attacking force. They assaulted a few English outposts and the English army retreated.

She had played no great part in the skirmishes but became a heroine for Charles' previously demoralised forces, who won a couple of serious battles. Charles was anointed king and then, on Jeanne's urging, marched on pro-“English” Paris and was utterly routed.

Charles VII rejected Jeanne's messianic zeal to “boot out the English” and sought diplomatic solutions. Disgusted, she fought a brief guerrilla war, with a small band of devotees. She was captured by the Burgundians outside Compiègne on 29 May, 1430.

King Charles ungratefully refused to ransom her. The English paid £10,000 and handed her to the church, which strongly disapproved of peasant girls who spoke to saints. After a lengthy trial in Rouen, in which Jeanne defended herself with great intelligence and dignity, she was condemned for heresy and witchcraft (partly for having worn a man's clothes). She recanted, to avoid the stake, but a few days later was “found” in her cell dressed as a man. For having “relapsed”, she was burned alive.

Joan herself claimed that her women's clothes had been taken away. She had to dress as a man or go naked. This was probably a medieval sting operation, conducted by anti-Joan elements in the French church and the University of Paris.

Revisionism can go too far. The legend of Joan helped to solidify the “French forces” after her death. She did therefore play some part in booting the English out of France. Her second, posthumous trial, which cleared her in 1456, was a symbol of the post-war creation of a first, true sense of French national identity.

The transcripts of her two trials mean that Jeanne's personality and voice have survived through the centuries: calm, obstinate, driven but not really the voice of a fanatic. We learn, among other things, that she was a wonderful cook and a good-looking woman with large breasts.

Jeanne d’Arc was plainly a creature of a 15th century in which people, recovering from the black death and beset by terrible wars, saw signs and omens all around them. She is also oddly modern and inspired writers from George Bernard Shaw to Mark Twain, Jean Anouilh and Bertolt Brecht because of the mental strength that she - an illiterate teenager - displayed in standing up to her malevolent inquisitors.

It is possible to dismiss Jeanne/Joan as a medieval fanatic. It is also possible to recognise in her the stirrings of belief in the individual conscience, one of the building blocks of western modernity. To make her a heroine for xenophobes is to diminish her. To recruit her, 600 years on, to a struggling president’s re-election campaign is ridiculous. She belongs to all of us.

National icons: Saint George

It is not only France that has a problem with far-right political parties appropriating national symbols as their own. England's patron saint is celebrated by right-wing extremist parties such as the British National Party (BNP), the English Defence League and the League of St George as a heroic symbol of the English ideals of honour and courage.

The BNP leader, Nick Griffin, was flanked by a party member dressed in a St George costume when he launched the party's manifesto on St George's Day in 2010.