Bastille Day is meant to be a moment of celebration in France. But when my wife and I had dinner on Thursday evening with neighbours near our French home in the Loire region, we encountered visceral despair about the state of the Gallic nation.

The company was charming, the hospitality magnificent, yet parts of the conversation were profoundly sombre. This was hours before the news of the Islamist atrocity in Nice emerged, but our friends' concern for France's future was palpable.

Mass immigration, the relentless growth of the Muslim population, the alarming spread of jihadism and the enfeebled stance of President Hollande's socialist government had left them with a feeling that their country is increasingly under siege.

WARNING GRAPHIC CONTENT

Terror attack: At least 84 people were killed when Mohamed Lahouaiej Bouhlel drove this truck, riddled with bullets, through crowds celebrating Bastille day in Nice

A child's doll lies on the street beside the body of a young girl who was killed in the attack in Nice in the south of France

Their dark forebodings were dramatically confirmed when we returned home and learned of the carnage on the Riviera. Another part of this beautiful land had been turned into an arena for butchery.

This week Patrick Calvar, the head of France's General Directorate for Internal Security, warned that his country is 'on the verge of civil war' because of growing tensions between Muslim communities and the hard Right, represented by Marine Le Pen's insurgent National Front Party.

In remarkably frank language to a French Parliamentary commission, Mr Calvar said that a serious incident could 'light the powder, transforming France into an uncontrollable country, where groups take up arms and hand out their own justice'.

The massacre in Nice gave a terrible resonance to his words.

This is just latest in an appalling catalogue of recent radical Islamist violence against the French, including the mass murder at a Jewish school in Toulouse in March 2012, the Charlie Hebdo killings in January last year, the Bataclan concert hall shootings in November, and the execution of a police officer and his wife in June.

It is no surprise that in response to this barbarism, the National Front, once regarded as an irrelevant if despised fringe group, is enjoying an unprecedented surge in support.

Its leader, Marine Le Pen — daughter of the party's founder Jean-Marie Le Pen — is certain to be a serious contender in next May's Presidential Election.

French police riddled the truck with gunfire in an effort to kill the crazed driver of the truck which ran down hundreds of people

Pictured: The lorry used as a murder weapon on the French Riviera city's famed waterfront promenade with police gathering evidence and marking bullet casings with yellow numbered signs

And with the mainstream establishment in disarray and concern over the threat of militants growing, it is not inconceivable that she could come close to winning.

Nice will almost certainly prove a great recruiting sergeant for her. 'Security' is now a favourite word among ordinary French people, encapsulating their fear that their society is collapsing because of bitter divisions.

I have had my own direct experience of this ever-growing tension, which threatens to tear apart the country I love.

More than a decade ago, long before we moved to the Loire region, my wife and I bought a 19th-century house in the heart of Carpentras, a Provencal market town with a population of 30,000, little more than a two-hour drive north of Nice.

The place had a rich history stretching back many centuries, the architectural legacy of which included a triumphal Roman arch and a magnificent gothic cathedral.

Our new home needed a lot of work, but the task seemed worth it because we could spend part of the year enjoying life in southern France. And at first our times in Carpentras seemed idyllic, wandering through market squares or sitting in a cafe under a cloudless blue sky.

Shootout: Police approach the truck which was used by gunman Mohamed Lahouaiej Bouhlel in the attack

Intense fighting: Police fired dozens of rounds into the cab of the lorry after its mile-long rampage with witnesses saying the suspect was firing out of the cab and died

But gradually, shadows began to creep across our retreat. What we had thought was a classic Provencal existence turned out to be something very different. Over the years, Carpentras underwent a dramatic change as the Muslim population grew and the town became ever more Islamified.

Although ethnic monitoring is illegal in France because it is seen as divisive and offends the concept of Gallic solidarity, it has been conservatively estimated there are at least 13,000 Muslims in the town, making up more than 40 per cent of the population.

Some have put the figure as high as 60 per cent. Two mosques, one of them a massive new block, have been established to meet the changing religious demographic. Inexorably, the streets were becoming filled with figures in Islamic dress, along with halal butchers and kebab shops.

In response to this transformation, the owner of the internet cafe opposite our house grew increasingly fervent in his support for the National Front, putting up large posters for Jean-Marie Le Pen in his windows, which were regularly smashed.

Throughout all this, we could sense that the gentleness of Provence, scented by grapes, lavender and sunflowers, was giving way to a mood of suspicion and latent threat.

A truck ploughed through a large group of people out enjoying themselves in Nice during the terrorist attack

End: The police said that French Tunisian Mohamed Lahouaiej Bouhlel died in the passenger seat of this 19-tonne truck after a shoot-out

One night I woke up to the smell of acrid smoke in the air. Looking out from my bedroom window, I saw to my astonishment that five cars had been set on fire in our street.

On another occasion, while out in the countryside with my wife, I was menaced by a Muslim armed with scythe.

When, slightly shaken, I told this to a neighbour, who was a French army veteran, he recounted how a local Muslim had one day threatened to slit his throat.

Let's be absolutely clear: most of the Muslim population were thoroughly decent people who wanted nothing more than to live their lives in harmony with other peaceful French people.

That said, the religious and racial tension in Carpentras was palpable in everyday life.

Carpentras has the oldest synagogue in France, and the town's Jewish roots were another source of this tension.

My wife and I went one night to a choral concert at the town hall by a renowned Israeli choir, but because of Islamist threats of violence, security was as tight as it might have been for a visiting foreign leader, complete with guard dogs and armed troops.

It was partly because of the death of our Provencal dream that we sold our Carpentras house three years ago and moved further north.

At the very moment of our departure in 2012, there was another indication of the rising local discord when Marion Marechal-Le Pen, niece of Marine, was elected the local deputy for the National Assembly, ousting the long-serving centre-Right incumbent Jean-Michel Ferrand, for whom I had done occasional work as a speech-writer.

Horrifying: Forensics teams inspect bodies in the French sunshine where 84 died and 50 were injured

Disaster zone: Bodies lie along the promenade showing the devastating route the lorry took as it went through huge crowds

Last year, she headed the poll for Provence in the first round of the regional elections, though she failed to win the seat.

'I'm not afraid of Marion. I'm afraid of terrorism,' said one of her voters when explaining the rise of this far-Right politician. 'We've had enough immigration. It's time to close the borders.'

And this sort of sentiment echoes across France. 'Things fall apart, the centre cannot hold,' wrote the poet W. B. Yeats of the Irish conflict in the early 20th century.

His words could also be applied to modern France, where there is a widening chasm between the indigenous, intensely nationalist French and the detached, often hostile, Muslim community, which is estimated to number anything between 4.5 and 7 million people, about 10 per cent of the total.

Thanks to the high Muslim birth rate, massive immigration from North Africa and the EU policy of open borders, that population is already the largest in Western Europe and growing all the time.

In response to the Nice atrocity, President Obama declared that 'we know that the character of the French Republic will endure long after this devastating and tragic loss of life'.

But how can he be so sure? The truth is that France, as many of us know and love it, may not endure because the very fabric of French society is changing so rapidly.

Terror investigation: The French authorities say it is their third terrorist atrocity in 18 months and admit it could take some time to identify all the dead

Pictured: Victims covered by sheets remain at the scene of the terror attack as the authorities try to identify them

At the heart of the potential confrontation is a clash of two different cultures. On one side, there is the French tradition of a powerful national identity and strong secularism, stretching back to the late 18th-century revolution.

On the other is the increasingly self-confident Muslim minority that refuses to accept the French ethos of assimilation. As we have seen in Nice, that spirit of often aggressive detachment is reflected at its most extreme in the growth of Islamic radicalism and terrorism.

Many commentators talk grandly of Isis and jihadism as if they were foreign problems that can be dealt with by military action in the Middle East. But the reality is that Islamic extremism in France is largely home-grown.

Indeed, the Nice mass murderer is a typical case. Though of Tunisian origins, the 31-year-old behind the massacre was a French national.

Muslim disaffection is displayed in other trends, like low participation in the jobs market — unemployment in Carpentras is double the French average — and the high rates of criminality that invariably go with joblessness.

Incredibly, more than 60 per cent of the French prison population is estimated to be Muslim, with the result that jails are now serving as a breeding ground for radicalisation.

But there are wrongs on both sides. The rising tensions are further reflected in the surge in hate crimes across France. While terrorism worsens, more than 400 anti-Muslim incidents, including assaults, harassment and criminal damage, were reported to the authorities last year, up from 133 such incidents in 2014.

Meanwhile, in just the first five months of 2015, 508 anti-Semitic crimes were recorded, an increase of 84 per cent on the same period the year before.

Aftermath: This photograph was taken in the moments after the lorry crashed through crowds of people in the devastating terror attack

Carnage: The killer zig-zagged across the road trying to hit as many people as possible, including several children, who were killed as they watched the fireworks

Until recently, violent anti-semitism in France was largely seen as the preserve of the far Right, a dark legacy of Vichy France's collusion with the Nazis during the war, but today it is another weapon of Islamist intimidation.

It is common — particularly among the Left — to blame these mounting social problems on discrimination, poverty and the aftermath of French colonialism in North Africa.

One of the most repeated theories for Muslim alienation in France is that immigrants from North Africa in the Sixties were 'dumped' in poor housing.

Vast concrete estates known as Les Banlieues (the suburbs) exist on the edges of cities throughout France, from Paris in the north to Marseille in the South. Dominated by Muslim communities, they have become symbols of Islamic grievance, and are seen as breeding grounds for disillusion and extremism.

But it is more complicated than this. Throughout the history of mankind, immigrants have had to put up with difficult conditions in their new lands.

Their success in overcoming their tough backgrounds — as with the Irish who came to Britain after the potato famine in the late 19th century, or the Italian migrants to New York in the early 20th — is part of their inspiring narrative.

Moreover, the provision of social housing and welfare were gestures of generosity towards newcomers, not rejection.

From my experience of living here, there is no institutionalised discrimination against Muslims in France. Why, otherwise, would Muslims star in the French national football team or at the top of French politics?

In the 2012 National Assembly elections, five MPs of Maghreb — or North African — origin were victorious.

84 people were killed in the southern French town of Nice when a truck ran into a crowd and targeted people on the pavement

The driver mounted the pavement at 40mph and steered directly at large groups of people watching the fireworks

Indeed, the whole thrust of French national culture is against discrimination, and there is considerable integration of successful Muslims.

Tellingly, statistics show France has the highest number of mixed unions between people of different religions and ethnic groups of any country in Europe.

But crucially, the country is also unashamedly patriotic as well as secularist, in the sense that ever since the Revolution in 1789, religion has been held to be a matter of private conscience rather than the state involvement.

This is why the French refuse to bow to the demands of uncompromising adherents of Islam who want special treatment. Why they ban the burka and insist that schools should be allowed to serve pork.

Inevitably, this approach inflames a sense of grievance among some Muslims, particularly those of North African origin who hark back to the country's colonial period and the savage Algerian war for independence in the Fifties and Sixties.

But I believe that if too many Muslims feel excluded from French society, it is because of their own decision to reject France's values and identity. Instead, they wish to pursue their own theocratic, separatist agenda.

To be fair, this is not a problem unique to France. Britain may have only 600 jihadists fighting in the Middle East, compared with France's 1,200, but that highlights the numerical truth that France's Muslim population is double the size of ours.

The fact is that, because of the nature of modern, militant Islam, wherever there is a significant Islamic population anywhere in the world, whether it be in Boston or Bali, there will be tension.

Shocked survivors held each other after witnessing the massacre which killed 84 people injuring at least 150 others

Soldiers secured the scene while emergency workers treated the wounded and checked on those killed in the massacre

The decision by Germany's Chancellor Merkel last year to invite more than a million Syrian refugees to Europe, combined with the EU's ideological but self-destructive obsession in abolishing border controls, has only made the problem worse.

A report this week by the U.S.-based Pew Research Centre showed that, in eight out of ten European countries surveyed, more than half of the population believe the influx of immigrants has increased the terror threat.

In fact Ronald Noble, the former head of Interpol, has said that the EU's open-door approach is 'like hanging a sign welcoming terrorists to Europe'.

With militant Islam now flourishing in France's midst, there are no easy answers. The sheer scale of the problem is daunting.

After the last Paris terror attacks, the police carried out more than 3,500 raids in Muslim-dominated areas, but that is probably just tackling the tip of the iceberg.

And the truth is the French intelligence services cannot cope. It would take 30 officers to provide comprehensive surveillance of each suspected radical, far beyond the resources of the French state.

Nor is there any sign of immigration abating. In our village in the Loire, some of the locals talk ominously of a social breakdown.