Macron, who had made integrity in office a cornerstone of his campaign, was forced to fire Alexandre Benalla, now 30, after a video emerged showing him striking a young man and grabbing a young woman by the neck at a May Day protest in 2018.
The former bouncer was wearing a police helmet, although he had only been given leave to accompany security forces as an observer.
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But the presidency held off on reporting the assault to the authorities, and it came to light only after French daily Le Monde revealed the existence of the video a few months later.
Officials denied allegations of a cover-up but “Benallagate” ballooned into the first major test for Macron, who swept to the presidency a year before with a promise of restoring an “exemplary republic”.
His government survived two no-confidence votes in parliament, but a Senate investigative panel that questioned key Macron aides found “major flaws” in the administration’s handling of the affair.
Benalla was charged with assault as well as unauthorised interference in police matters and use of police insignia.
He claims he acted “by reflex” to help officers arrest unruly protesters.
Also on trial is Benalla’s friend Vincent Crase, the former security head of Macron’s centrist party, who was also filmed roughing up protesters at the demo.
Two police officers, accused of illegally providing Benalla with surveillance footage in a bid to claim his actions were justified, will also be in the dock.
Benalla’s lawyer declined to comment ahead of the trial.
“Apprehending someone who is committing a crime is not punishable by law,” Crase’s lawyer Christian Saint-Palais told AFP, saying his client reacted “spontaneously”.
Benalla started working as a bodyguard for Macron in 2016 during what many considered his dark horse bid for the presidency, winning over the politician and his wife Brigitte with his boundless energy.
He was promoted to a senior security role after Macron’s May 2017 victory, becoming a trusted confidant and right-hand man seen at Macron’s side in countless photos.
“He got noticed as someone who could fix all the practical problems very efficiently,” a former senior campaign official said.
“He thought of everything, he was our Swiss Army knife.”
He also secured perks usually reserved for top administration officials, including an apartment near the Elysee Palace and access to the National Assembly and its private gym and library.
After the scandal broke, Benalla also admitted carrying a handgun during outings with Macron, even though he was only authorised to have it within Macron’s party headquarters, where he was nicknamed “Rambo”.
It remains unclear, however, how the young man from a working-class neighbourhood of Evreux, a sleepy town in Normandy, got hold of the police helmet he is seen wearing during the May Day assault.
Benalla also caused headaches for Macron after his dismissal.
Investigators found that he continued to use diplomatic passports for trips to Africa and Israel, where he was trying to build up a consulting business.
He is suspected of using faked documents to obtain one of the passports, a charge he has denied.
Benalla will also face a charge of illegally carrying a gun, based on a picture of him at a restaurant that appears to show him with a Glock.
He has said it was a water gun.