Think what your enemy wants you to do, and do the opposite. No maxim of war is so ignored.

Since last Friday’s killings in Paris, the world has answered the dreams of Islamic State. It has drenched their deeds in fame, glorified its perpetrators with vilification and defined them as warriors not murderers. Deeds of the most squalid horror have been “nationalised” then internationalised. The whole world has been drawn into Isis’s web of fear. Its wildest fantasies have been realised.

The potency of terror lies not in the act but in the aftermath. The act is death and destruction, horrendous in itself. The response is what gives it political traction. As with Osama bin Laden on 9/11, Isis wants the world to go berserk, declare emergencies, tear up freedoms, persecute moderate Muslims and bomb Muslim cities. By capitulating to these desires, the west has vastly increased the power of the terror – and the likelihood of imitation.

Western leaders seem blind to reason. The pope has spoken of a third world war. David Cameron cites Hitler and the Nazis. Both references suggest scant knowledge of history. Extra money is hurled at military hardware. The French are bombing the Isis capital, Raqqa, doubtless proving that bombs from the air kill the innocent as effectively as guns on the ground.

Every western lobby against freedom is rushing from the shadows, knowing its hour has come. The British home secretary, Theresa May, rightly warns that “changing our way of life” is to play into the terrorists’ hands. Yet she demands surveillance powers that do just that, while her boss compares himself to Churchill and hurls £2bn at “special forces equipment”. Special forces have no relevance to suicide bombers. The need is for human intelligence within Muslim communities, at which Britain has proved remarkably successful.

Bin Laden showed that mass killing is the easiest fuel for the politics of fear, a fear that distorts democratic judgment, breeds overreaction and creates new and more terrible enemies. Human atrocity needs grieving and sympathy. The Paris massacre was a tragedy. It should not have been turned into an act of war.

War is a conflict between states. The killings did not threaten the integrity of any state. That threat comes only from how we respond, by playing the terrorist’s game. This week’s treatment of Isis validates its claim to be waging jihad. Why should the west offer it victory, when we should be strong enough to offer quiet contempt?