PARIS • Ever since the terrorist attacks in January on the satirical magazine Charlie Hebdo and a kosher supermarket, Parisians knew that barbarism lurked around the corner and that it would strike again.
But it is one thing to know something, to anticipate it, and another to be confronted with the grim reality.
Last week, reality struck us with a vengeance. We are at war. It would be wrong - even dangerous - not to admit it. And to win will require clarity, unity and firmness.
Clarity of analysis is what we now need the most. We barely know our enemy, except for the intensity of his hatred and the depth of his cruelty.
To understand his strategy, we must recognise him for what he is: An intelligent - and, in his own way, rational - adversary. For too long, we have despised and underestimated him. It is urgent that we now change course.
In the last few weeks, the Islamic State in Iraq and Syria's (ISIS) strategy of terror has brought death to the streets of Ankara, Beirut and Paris, and to the skies over Sinai.
The identities of the victims leave no doubt about the message. "Kurds, Russians, Lebanese Shi'ites, French: You attack us, so we will kill you."
The timing of the attacks is as revealing as the targets' nationality. The more ISIS is defeated on the ground and loses control of territory in Syria and Iraq, the more it is tempted to externalise the war to deter further intervention.
The synchronised attacks in Paris, for example, coincided with ISIS' loss of the Iraqi city of Sinjar.
Of course, the terrorist cell that struck Paris was not created in the wake of ISIS' recent battlefield losses. It was already in place, waiting to be activated (as others may be). That demonstrates ISIS' tactical flexibility, not to mention the availability of people willing to commit suicide.
If ISIS chose this time, in Paris, to target people who are not satirists, policemen or Jews, it is precisely because their "ordinariness" left them unprotected.
This time, the attackers chose "quantity" over "quality" (if one may be pardoned for such a crude formulation). The goal was to kill as many people as possible.
This strategy is possible because the territory controlled by ISIS provides a sanctuary and training ground. The self-proclaimed caliphate's territories represent for the group what Taleban-controlled Afghanistan meant for Al-Qaeda in the 1990s.
It is imperative to regain control of this territory. And destroying ISIS' "provinces" in Libya, Sinai and elsewhere, must become the number one priority of the international community.
Beyond analytical clarity, there is a need for unity, beginning in France, where citizens would reject their political class were its members to continue to behave divisively at such an obvious historical turning point.
Unity must also be achieved within Europe. We are repeatedly told that Europe is in the midst of an identity crisis, in need of some new project. Well, now Europe has found one.
To be European means to confront together the scourge of barbarism, to defend our values, our way of life and our way of living together, despite our differences.
Unity is also required of the Western world as a whole. President Barack Obama's statement after the Paris attacks demonstrates that what unites Europe and the United States is much more significant than what divides us.
We are in the same boat, faced with the same enemy. And this sense of unity must go beyond the European and Western world, because ISIS threatens countries such as Iran and Russia, not to mention Turkey, as much - if not more - than it does the West.
Of course, we must be realists. Our alliance of circumstance with these countries will not overcome all problems between them and us. So, beyond clarity and unity, we need firmness, both in confronting the threat of ISIS and in defending our values, especially adherence to the rule of law.
ISIS expects from us a combination of cowardice and overreaction. Its ultimate ambition is to provoke a clash of civilisations between the West and the Muslim world. We must not fall prey to that strategy.
But clarity comes first. When Paris is attacked, as it was last Friday, one must speak of war.
No one wants to repeat the errors of the US under President George W. Bush; but to use those errors as an alibi to avoid confronting the world as it is would merely be an error of a different sort.
Europe's response must be tough, but it must not deviate from the rule of law.
We are, after all, engaged in a political battle with ISIS, one in which our love of life must prevail over their love of death.
• Dominique Moisi, a professor at L'Institut d'études politiques de Paris (Sciences Po), is Senior Adviser at the French Institute for International Relations (IFRI) and a visiting professor at King's College London.
