Bangladesh, March 16 -- The violent death of 23-year-old activist Quentin Deranque in the city of Lyon has shaken France far beyond the tragic loss of a single life. What began as a street confrontation between rival political activists quickly escalated into a fatal assault, leaving the young New-Right militant beaten to death after being cornered by individuals identifying themselves as “antifascists.”

At first glance, such clashes between radical groups might seem like an unfortunate but isolated incident in a country with a long history of ideological street politics. Yet the political aftershocks from this killing have been extraordinary. The episode has exposed deep fractures within French society, intensified partisan ...