FRANCE is at war,” said Francois Hollande on the morning after the worst attack on French soil since the Second World War. But at war with what? Islamic State isn’t a country. They don’t invade with armies, but with fear.

You can’t go to war with an organisation that doesn’t stand and fight and nor can you punish people who’ve already sacrificed their lives.

Hollande said the perpetrators will be pursued “without mercy”.

But you can’t sentence a suicide bomber to death.

IS are terrorists whose objective is not to occupy but to polarise; to encourage repressive measures from the state against Muslims, and to force non-Muslim communities to regard followers of Islam as “enemies within”.

They know that the influx into Europe of large numbers of migrants from North Africa and the Middle East is reawakening latent xenophobia in French society.

IS are doing a recruiting job for the extremist Front National of Marine Le Pen who is expected to win next month’s regional elections in northern France.

The FN wants Europe to rebuild its borders and end free movement.

The Paris attacks seem to have been consciously targeted at trendy cafes and restaurants attended by urban liberals who celebrate multiculturalism and for whom religious or racial intolerance is abhorrent.

It was retaliation against the Paris that came out in force to express solidarity with the victims of Charlie Hebdo in January.

“They curse our prophet,” said the IS statement. The rhetoric may be mediaeval but the tactics are 21st century.

These are digital zealots, connoisseurs of popular culture, who may even posses a grim sense irony.

Their main attack in Paris was at a rock concert fronted by the American band Eagles of Death Metal.

But it was real death metal flying into the bodies of young people from Kalashnikovs wielded by Islamist fanatics.

The message was clear: the young people in the West play at death; IS do the real thing.

IS says it is targeting Paris in part because of the bombing in Syria, but principally because it is “the capital of adultery and vice”.

The cover of the Eagles of Death Metal’s latest album Zipper Down depicts a woman in a leather jacket revealing her breasts.

The imagery will not be lost on the legions of young impressionable Muslims on the internet.

The only way to defeat IS is to withstand it. The people of Paris understand this.

On BBC radio yesterday a young Parisian announced that “everybody is going to go out and eat cheese and drink wine like we always do on Saturdays”. That’s the spirit.

Hurling rhetoric and more drones at IS only makes it stronger. The best way to combat this kind of threat, is to keep calm and carry on.

That’s how Britain withstood the IRA’s mainland bombing campaign which killed so many.

The one thing the terrorists want is for governments to launch another war on terror, just as America did after 9/11. So let’s hear no more of it.

The weapon Islamic extremists fear most is tolerance.