Trying to make sense of terrorism is complicated for ordinary citizens: What are the aims of those behind it? Why do they do it? What makes someone willing to carry out attacks, often suicide attacks?

But there are some things about it that are simpler to explain.

Why Barcelona? Why Cambrils?

A key to making sense of this was recently handed to me by the owner of a political marketing agency: "Terrorists seek impact." Though it might sound inappropriate in this context, in advertising jargon the term "impact" is used when a message reaches its target audience.

With this in mind, one can make sense of the choice of Barcelona and the coastal town of Cambrils as targets for attacks. For years, Spain has been in the crosshairs of jihadi extremism; several experts thought it was just a matter of time before events like those of August 17 took place. But by choosing Barcelona and, specifically, one of its busiest landmarks and tourist attractions, the objective was to maximize the number of people on whom the attack would have an "impact." How many Spaniards have relatives in Barcelona? How many tourists thought to themselves: "I was on the Ramblas during my last holidays; I could have died there?"

That is what the terrorists are aiming for: kill a few, but scare millions. And they are doing it gradually, because, unlike what they would have us believe, their weapon of choice - fanatics willing to die in the act - are not all that numerous. They will kill again. Perhaps not today, or tomorrow. But the moment we start to forget about the fear...

Helping Islam expand?

Winning notoriety, spreading their "achievements," sowing fear, recruiting: This is part of what terrorists want to achieve with their attacks. Those who commit them - often young, marginalized men seeking redemption - perhaps really believe they will go to heaven and receive their reward. Maybe they are just blinded by rage. But they are only a symptom, not the actual disease. What do those who foment jihadi terrorism want? Why is that those who preach hate, whether from the mosque pulpit or on the internet, are never the ones who wear the explosive belt?

If their final objective were to promote the expansion of Islam in the West, this would not be the best way. No: What they truly want is to create tensions, to stir up hatred against Muslims, to radicalize non-Muslims. Because the more they provoke rejection of Islam in the West, the more people they can recruit.

The final objective is to draw the world into a future war - one that they are convinced they will win, with Allah's help. But let's not fool ourselves: Jihadi extremism kills more people in Muslim-majority countries than in the West. Terrorism kills more innocents in Iraq and Afghanistan than in the rest of the world. Our own dead have a bigger "impact" on us, of course.

'The West is not their real target'

As Federico Aznar of Spain's Institute for Strategic Studies (IEEE) has said: "The West is not their real target, just part of their rhetoric. Western victims are for show; they are high-profile targets, an expression of power and resolution meant to bolster the group's legitimacy in their own territory, where their true interests lie." Terrorism is a political weapon. It has been used by nationalists, anarchists, communists, fascists and many other groups. Some of them - occasionally - reached their goals.

Let us not play into their hands. Let us not overestimate their capabilities. Let us not allow ourselves to be contaminated by their rage. Let us be vigilant. Let us keep an eye on returning jihadi fighters who have been immersed in extremist messages.

But we must not let them drag us into their perverse logic. Because, apart from the blood that has been spilled, the most harm terrorism can do is separating us from our values, from what we are: secular, open and tolerant societies in which religion is a strictly private matter. As Antoine Leiris, whose wife was killed in the attack on the Bataclan concert hall in Paris, put it: "You will not have my hatred." At least not hatred toward those who also condemn and reject these kinds of attacks. Allow me to save a little for those who carry them out. I only hope that this hatred does not grow too much.