WE ARE all Parisians now, on the front line of a fight not just for the existence of our societies, but for the liberty to live as we choose.
As landmarks around the world are illuminated with the red, white and blue of the French flag and hearts go out to those sacrificed on the altar of an evil and perverted ideology, uncomfortable truths about the enemy within must be faced.
International solidarity with France in its hour of grief and shock at the atrocity visited upon Paris on Friday night provides only a partial answer to the challenges faced by Western democracies.
Stepping up military action against the fanatics of the so-called Islamic State in the Middle East needs to happen, and is likely to involve some unpalatable compromises such as forging alliances with an increasingly anti-western Russia or reaching an accommodation with the repellent Syrian dictator Bashar al-Assad.
But a battle needs to be won closer to home, not only in Britain, but particularly in continental Europe, which has sleepwalked into becoming fertile ground for Islamist extremism to take root and grow.
What has emerged over the past three days is that the terrorists responsible for the massacre were citizens of France and Belgium, although their allegiance was not to those countries, but to the murderous fanaticism of Syria.
Their radicalisation appears to have taken place in slum neighbourhoods where a toxic mix of isolation, cultural separation and resentment has been allowed to fester by countries which have failed to address the challenges of integrating new arrivals.
Open borders across Europe which have allowed those who would murder to move weapons and explosives with impunity, and the perils of uncontrolled immigration, have been gifts to extremists.
They have preached their creed of hatred in run-down suburbs where new arrivals have congregated, areas too often conveniently ignored by countries which have regarded them as a problem which if left to itself will hopefully go away.
Instead, a culture of Islamist fundamentalism and alienation has developed and been exploited by terrorists.
The liberal values that Europe lives by – its cultural, political, religious and sexual freedoms – hold no sway in these ghettos where such expansive and inclusive attitudes are anathema.
If anyone doubted that the Paris attacks were a full-scale onslaught on western liberties, they need only swallow their disgust in order to read the statement from Islamic State claiming responsibility.
In it, the mostly young people massacred at a rock concert were branded “idolaters” and their evening out called “a party of perversity”.
Such warped and repugnant views are not just intended to cheer the killers on the ground in Syria, but fan flames of hatred in the breeding grounds of terrorism in Europe.
A few years ago, I inadvertently strayed into one such area on the outskirts of Paris when a friend and I became hopelessly lost and found ourselves driving through one of the grimmest places I have ever been.
It was a rabbit warren of narrow, dirty streets lined by broken-down tenement buildings, shuttered shop-fronts and aimless young men hanging around.
The sight of two white men in a car was unusual enough for people to stare as we drove by trying to find the main road.
There was no alternative but to stop and ask directions. But speaking workaday French was no more use than using English, as I found nobody who spoke it.
The atmosphere was sullen, bordering on hostile. Some turned their backs as I approached, others just shrugged, and a few resorted to intimidating stares. We were plainly not welcome. When I got back into the car, I locked the doors.
The Paris that the occupants of those squalid blocks of flats lived in might have been a different city from the one where the busy restaurants and bars were attacked on Friday night.
Britain has been much better at integrating immigrants and reaching out to communities, but even so we know to our cost that home-grown radicalisation and terrorism is a serious threat.
The suicide bombers from Yorkshire who murdered commuters on London’s transport network in 2005 are testament to that, as was the Prime Minister’s statement yesterday that seven terror plots have been foiled in the past six months.
Paris’s tragedy tells us that we are caught up in a fight on several fronts. In the skies of Syria and Iraq certainly – and possibly on the ground as well – but the hardest and most prolonged battles will be much closer to home.
The days of open access and uncontrolled immigration in Europe are surely numbered. But, most crucially, the fight will have to be taken to Europe’s meanest streets.
This is a war of ideas, far more complicated than just a battle between two sides, for our enemy is not a standing army, but a movement, and capturing hearts and minds from an implacable enemy will prove much harder than simply taking prisoners.