On Nov. 13, 2015, multiple gunmen and suicide bombers killed 130 people and injured 368 across six locations in Paris, France. Shortly after the attacks, ISIS claimed responsibility, and French President François Hollande declared the attacks an act of war and increased France’s bombing campaign in Syria.

In the wake of such a tragedy, it’s important to try to understand why it happened and what can be done to prevent future attacks.

Rationally speaking, the attack on Paris makes no sense from an organization whose authority comes from maintaining territory in its “caliphate”; provoking a major Western power seems like suicide.

ISIS believes itself to be the next caliphate, a literal Islamic State first started by Muhammad and later continued by supposedly worthy successors.

As a caliphate, ISIS believes it has absolute authority in the Muslim world, and that all Muslims must travel to Raqqa, ISIS’ self-proclaimed capital, and submit to the new caliphate. Despite its claims, most Muslims consider its caliphate to be illegitimate.

ISIS’ radical extremism requires it to constantly expand and to kill anyone whom it considers an enemy.

ISIS wants the West to invade, wants the fighting to take place in their territory, as they believe that they have a divine guarantee to win.

What this all means is that ISIS is an organization that cannot be defeated with normal means. ISIS members consider it a victory if they claim territory or if they die trying.

Until recently, ISIS has been suffering. The West’s refusal to engage in open war, combined with a slow loss of territory, such as the villages surrounding al-Hawl in eastern Syria or Ramadi, the capital of the Iraqi providence of Anbar, have led to decreased morale and infighting in the organization.

It’s entirely possible this attack was an attempt to boost morale or to prove that ISIS was capable of taking on a major Western power.

This leaves the West in an interesting position. To send in ground troops to invade Syria would be exactly what ISIS wants. ISIS contends that the West consists of a bunch of “crusaders” bent on eliminating the Islamic faith. To invade the Middle East again would support that narrative and possibly lead to increased recruitment.

ISIS’ ultimate weakness, however, lies in its extremism. Unlike al-Qaeda, which could simply go underground in the case of major losses, ISIS draws all of its authority from supposedly being the legitimate caliphate. In order for the caliphate to remain legitimate, it must continue to expand territory, wage jihad on enemies and assert its dominance on its inhabitants.

To this end, the West’s current strategy of bleeding ISIS out via bombing runs seems the best of all the bad options. By denying what the terrorists want, by slowly letting their radicalism tear them apart, the West will have a better chance of eliminating them, even if it may unfortunately lead to more foreign attacks.