Former President Nicolas Sarkozy of France on Monday went on trial in Paris over accusations that his 2007 campaign received illegal financing from the Libyan government of Col. Muammar el-Qaddafi.

The trial, which is scheduled to last three months, is far from the first for Mr. Sarkozy, 69, a conservative politician who led France from 2007 to 2012, but it represents one of the most serious legal threats to the French politician since he left office.

Just last month, Mr. Sarkozy exhausted his final appeal in a separate corruption and influence peddling case, making him the first former French president sentenced to actual detention, though he will serve his time under house arrest with an electronic bracelet.

But of all the legal cases against Mr. Sarkozy, the Libya one is among the most sprawling, convoluted and explosive. It involves accusations that his campaign illegally accepted vast sums of money from Colonel Qaddafi, the former Libyan strongman who was killed by opposition fighters in 2011.

Mr. Sarkozy, who arrived in court without making any comments, has denied wrongdoing. He could face up to 10 years in prison and be fined nearly $400,000.

Here is what you need to know about the case.

What is the trial about?

Mr. Sarkozy is facing charges of illegal campaign financing, criminal conspiracy, concealing the misappropriation of public funds and passive corruption (a charge that applies to people suspected of receiving money or favors).

The case against him involves a complex web of political and financial ties between Mr. Sarkozy’s advisers, officials who were part of Colonel Qaddafi’s government, and businessmen or bankers who acted as intermediaries.

Twelve other people were also ordered to stand trial on similar corruption, embezzlement or illegal campaign financing charges.

“Our thesis is that of a corruption pact,” Jean-François Bohnert, France’s top financial prosecutor, told RMC radio on Monday.

Prosecutors say that Mr. Sarkozy and his allies sought financing from Libya, in violation of election funding rules, and that the Libyan government promised to provide it. In return, they said, it wanted economic deals, diplomatic recognition and possibly assistance from France in rescinding an arrest warrant against a top Libyan official.

Mr. Sarkozy visited Libya shortly after he was elected, then welcomed Colonel Qaddafi for a widely-criticized state visit in Paris, where the Libyan strongman memorably pitched his Bedouin-style tent.

How did the case start?

In 2011, as Libya was roiled by fighting between the army and rebels, Colonel Qaddafi and his son said in media interviews that Mr. Sarkozy’s 2007 presidential campaign had taken Libyan money.

Then, in 2012, the investigative news website Mediapart published a document, presented as a note by Libya’s secret services, that mentioned a deal to fund Mr. Sarkozy’s campaign with up to 50 million euros, or about $52 million. That same year, as part of a separate investigation, Ziad Takieddine, a French-Lebanese businessman, made a similar allegation.

In 2013, prosecutors opened an investigation. It lasted a decade and involvedover 20 countries, 50 police raids and 70 volumes of case files.

How has Mr. Sarkozy responded?

Mr. Sarkozy has repeatedly and strenuously denied the accusations, which he argues were driven mostly by allies of Colonel Qaddafi seeking revenge.

Under Mr. Sarkozy’s leadership, France played a prominent role in the NATO-led campaign of airstrikes that ultimately led to the toppling of Colonel Qaddafi and his death at the hands of Libyan rebels.

There have been conflicting accounts about the sequence of events and the amounts of money involved, and some of the defendants have shifted their versions of what happened.

Some Libyan officials have even denied that Mr. Sarkozy’s campaign received any funding, and Mr. Sarkozy’s legal team has seized on the vagaries of the case.

“We don’t even have the amount of this alleged illegal financing,” Christophe Ingrain, Mr. Sarkozy’s lawyer, told RTL radio on Sunday. “Sometimes it’s in euros, sometimes in dollars, sometimes in dinars, sometimes 2 million, 3 million, 50 million, 400 million. This isn’t serious.”

Mr. Sarkozy’s official records for the 2007 campaign indicated that he spent over €21 million, and any illicit financing from Libya would have enabled him to skirt France’s strict spending cap for presidential campaigns. Prosecutors have not clearly laid out how much Libya actually sent or how much they believe was actually spent on the campaign. But under French law, prosecutors do not have to prove that a corrupt deal was carried out to secure a conviction — only that one was agreed upon.

Mr. Sarkozy no longer holds public office. But his memoirs are best-sellers, he is still popular with the base of his conservative party and he retains some political influence.

Has Mr. Sarkozy been convicted before?

Yes, twice. Mr. Sarkozy has faced multiple accusations of financial impropriety since he left office.

In 2021, he became the first former president in France’s recent history to be sentenced to actual detention after he was convicted of trying to obtain information from a judge about a court case against him.

Mr. Sarkozy has exhausted his appeal options in that case, but he will not be incarcerated. Instead, he will serve one year under house arrest with an electronic bracelet, although a judge has not yet ruled on the practical details.

Mr. Sarkozy was also convicted in 2021 to a year of house arrest for illegally financing his unsuccessful 2012 re-election campaign, which wildly exceeded France’s spending limits. An appeals court last year upheld the conviction but halved his sentence, and that case is still going through the appeals process.

Other cases against Mr. Sarkozy have been dropped, including one in which we was accused of manipulating the heiress to the L’Oréal fortune into financing his 2007 campaign.

And some cases are still being investigated, including an offshoot of the Libya case. In 2023, Mr. Sarkozy was placed under formal investigation on charges of witness tampering, after allegations that his allies pressured Mr. Takieddine, the French-Lebanese businessman, into retracting his accusations.