The Clearstream Affair, Act 1 Scene 1

It's murky and it runs deep; it's called L'Affaire Clearstream. It's been going on for five years and now it's being played out in court for every French citizen to see. From day one in court, L'Affaire appears trickier and more tortuous than an early Chabrol film.

To start with, what is L'Affaire about? A case of paranoia, slander and vengeance involving: a) Dominique de Villepin, an ex-prime minister who dazzled the world on 14 February 2003 with a historic speech at the UN against the war in Iraq, a Gaullist with a taste for history and poetry and a penchant for Bonaparte; b) Nicolas Sarkozy, former Chirac minister, today president of France, whose permanent agitation has transfixed his compatriots, and amused, irritated and awed the world in equal measure since his election on 5 May 2007; c) the French intelligence services.

You know the actors, what about the plot? In short, a secret list supposedly originating with the Luxembourg bank Clearstream appeared to show that many French politicians, among them Sarkozy, held secret bank accounts set up with money laundered through secret arms deals. The allegations were false, and the accounts did not exist. Problem is, De Villepin is accused by Sarkozy of having weighed in for the case to be pursued even though he knew the list to be false. Sarkozy is thus the key plaintiff in this affair, although there are another 40 on the fake list who are suing the secret services for slander. De Villepin, while he has plenty of company as a defendant, is the key accused.

It is, in fact, a case of political and personal hatred gone too far: De Villepin v Sarkozy. In 2004, Prime Minister De Villepin was hoping to damage the reputation of his direct opponent in the presidential race; in 2009, President Sarkozy wants to crucify his former rival, or rather in his own words, hang de Villepin "on a butcher's hook". Sarkozy is known for his rancour and unforgivingness; here is one more example of how far he is willing to go with a grudge.

All would be fine were Sarkozy not president. De Villepin's meddling in 2004 seems quite probable. But it remains to be seen how involved he was. And if guilty, he should be reprimanded for this. Today, however, Sarkozy's duty as president is to turn the page. Otherwise, he will be seen – as is already the case – as having vengefully brought pressure to bear on the judiciary by forcing a trial. Furthermore, the inequity of treatment between De Villepin in the dock and a president who enjoys immunity from prosecution doesn't serve Sarkozy's image.

The trial, which should take a month, will no doubt deliver a daily stream of revelations, piques, tirades, bons mots and, perhaps, even its coups de théâtre, but its opening has already seen the once almost-forgotten De Villepin welcomed in the court as a hero by the public, some of whom had queued since 4am to be there. The majority of French people tend to think that this affair should never have gone this far. If one reads former prime minister Édouard Balladur's forthcoming book on his "cohabitation" with François Mitterrand, one realises that blows below the belt are stock-in-trade for politicians. But not every coup should be settled in court.

To be continued …

Agnès Poirier, a political commentator and film critic for the British, French, Italian and Polish press, and a regular contributor to the BBC on politics and films.