If you haven’t yet read my TAC colleague Scott McConnell’s excellent primer on the political and cultural climate in France on the eve of its election,  please do. As Scott puts it:
Think what you will about America’s contentious identity politics; compared with France, the United States remains Mayberry, TV’s symbol of small-town innocence. We may have Black Lives Matter, massive resistance to a president seeking to enforce the country’s existing immigration laws, and urban riots. But in France the riots are bigger and last far longer. It has hundreds of thousands of people possessing French citizenship but evincing no discernible national loyalty. And there are few geographic barriers between itself and the sources of inundating immigration. No one can forecast with confidence the American future—whether it be a more or less successful assimilation of large streams of new immigrants or a transformed country where ethnic division is a norm underpinning every political transaction. But whatever the fate of Western civilization—whether it be a renaissance, or, as Pat Buchanan has predicted, its death—that fate will be revealed in Paris before New York or Chicago.
Note this passage especially:
Last year Michel Gurfinkiel weighed conflicting estimates (between three and six million) of the number of French Muslims in the mid-1990s and contrasted them with present estimates. He concluded that the current figure is roughly six million, or 9 percent of the population, and that it is growing at a much faster rate than the French population as a whole. As early as 2010, fully 20 percent of French under 24 were described as Muslim. A more recent poll in the liberal French weekly L’Obs reported that more than a quarter of French youth described themselves as Muslim.
Because the government does not publish statistics about race, some curious researchers have looked at the number of newborn babies screened for markers for sickle-cell anemia, a test given if both parents are of African, North African, or Sicilian origin. The figure has risen from 25 percent in 2005 to 39 percent in 2015. In the Greater Paris region it has risen from 54 percent to 73 percent. One understands why Houellebecq’s right-wing professor says he wants the inevitable civil war to come “as soon as possible.”
This article from the NYT  touches on the impossibility of the French police monitoring every French person on its radicalism watch list:
Jean-Charles Brisard, the chairman of the French Center for the Analysis of Terrorism in Paris, called the idea “absurd” and said France could not jettison civil liberties.
He added that putting everyone on the S List under surveillance was impossible, because there are more than 10,000 names and fewer than 5,000 agents. It takes 20 agents per suspect for 24-hour surveillance, he said, meaning France could perform round-the-clock surveillance of only a small fraction of those suspected of being radicalized.
“My profound conviction is that unfortunately we need to get used to living with this new threat,” Mr. Brisard said. “It’s permanent, it’s diffuse and it can erupt at any moment.”
You begin to see why ordinary French people speculate about a coming civil war within France.
In the wake of the shooting Le Pen called for foreign terror suspects to be expelled immediately and said it was a ‘ceaseless and merciless war’ against France which required ‘a presidency which acts and protects us’.
The killer of the Champs-Elysées was French-born, but Le Pen surely understands that expelling French citizens is not possible. But if France can expel those radicals without a legal right to remain in the country, it should, whether or not they have been convicted of a crime.
Even if that radical step were to happen, it would only put a dent in the problem. The C-E killer was, as I said, French-born, but he was not on the S List (the government’s terrorism watch list), even though he had served a prison term for trying to murder police officers: 
The attacker, a 39-year-old Karim Cheurfi, was known to French security services. Media reported he had served nearly 15 years in prison after being convicted of three attempted murders, two against police officers, and was released on parole in 2015.
The attacker was shot dead by police in the van while trying to flee the scene on foot. A statement from the Isis propaganda agency, Amaq, said the attack was carried out by an “Islamic State fighter”.
… A house in the eastern suburb of Chelles, believed to be Cheurfi’s family home, was being searched on Friday. Le Parisien newspaper said the address matched that of the owner of the car used in the attack.
Police found a pump-action shotgun, knives and a Qur’an in the vehicle, while a handwritten note praising Isis was later recovered near the dead attacker, police sources told local media.
They said Cheurfi was arrested in February on suspicion of plotting to kill police officers but released because of lack of evidence. He was reportedly not, however, on France’s “Fiche-S”, the list of people suspected of being a threat to national security.
The threats came at an unusual turn in Kepel’s career. He has long been a prominent figure in the French intellectual world, a scholar whose face — a distinctive, narrow-eyed mask of polished sobriety — is often seen on TV news shows. But recently he has assumed a far more combative stance. Kepel has argued that much of France’s left-leaning intelligentsia fails to understand the nature of the threat the country faces — not just from foreign terrorists but also from the Islamist provocateurs in its exurban ghettos, the banlieues. Unlike the Islam-bashing polemicists who haunt French opinion pages, Kepel brings a lifetime of scholarship to this argument. He has always been careful to distinguish mainstream Islam from the hard-line Islamist ideologues of the banlieues, who have no real equivalent in the United States. He has long been a man of the left; his wife’s family is from North Africa, and he has no sympathy for the xenophobia of the right-wing National Front. But he believes that radical Islamists are trying to shred France’s social fabric and foster a civil war, and that many leftists are unwittingly playing into their hands. This view has made him a target for almost everyone.
One of the most common critiques of Kepel is that his relentless focus on Islam casts a shadow of suspicion onto all French Muslims. As [Olivier] Roy put it to me, “If you say it’s a religious issue, then the extremists are seen as the avant-garde of the whole Muslim population.” Jean-Pierre Filiu, another prominent French scholar of the Islamic world, pointed out that several thousand Muslims marched for peace in Mantes-la-Jolie after the Abballa murders, many of them bearing pictures of the murdered couple and posters denouncing terrorism, and laid wreaths on the steps of the local Police Headquarters. There was no one there to greet them, and not much news coverage. “The jihadis want to blur the lines, but the lines should be clear,” Filiu told me. “It’s not the Salafis who are against us, and not the Muslims. It’s the jihadis.”
These are generous sentiments, and no doubt many French Muslims appreciate them. Kepel would say they seem less aimed at truth than tact, the idea that hurting Muslim feelings will poison the atmosphere further. At its extreme, this view risks its own form of condescension: Be nice to Muslims or they will turn into suicide bombers.
Kepel has argued in his recent books that the French Muslim community, once guided by the paternalist figures from the old country known as darons, is now increasingly under the sway of younger and far more confrontational Islamists. These ideologists, Kepel believes, have fostered a rupture with French values that nourishes the ISIS narrative. Yet some French intellectuals naïvely disregard or even embrace these figures in the hopes of “isolating the radicals.” In other words, Kepel turns the accusation of Filiu and Roy — that his own emphasis on Islam is unwittingly doing the work of ISIS — against them. Kepel likes to cite ISIS propaganda urging its followers in Europe to hide behind the language of victimhood, including one document shared among ISIS sympathizers titled “How to Survive in the West,” which includes the following lines: “A real war is heating up in the heart of Europe. … The leaders of disbelief repeatedly lie in the media and say that we Muslims are all terrorists, while we denied it and wanted to be peaceful citizens. But they have cornered us and forced us into becoming radicalized.”
With all this attention focused on them, many jihadis are now adapting, and have become far better at disguising their beliefs. Farhad Khosrokhavar, a sociologist who has spent many years researching Muslims in the French prison system, told me it has become almost impossible to get honest testimony out of the inmates. Many of them shave their beards, Khosrokhavar said, and adopt a mild demeanor, and sometimes they even stop praying and fasting during Ramadan, all so as to deceive the authorities and, presumably, get out of prison faster.
… Just before we left, I asked the North African [Muslim prisoner] whether he expected the recent wave of terrorist attacks in France to continue. This was just after the arrest of several terrorist cells, and two months before a machete-wielding jihadist attacked guards near the Louvre. He gave me a somber look. “This is just the beginning,” he said.
Read the whole thing.  The author talks to French Muslims who actually agree with Kepel, and say that the real problem is the spread of Gulf-sponsored Salafism among French Muslims.
Poor France. Like Scott McConnell said, France is in the vanguard of issues that will eventually confront most Western nations.