The Islamic State group continues to pose a serious challenge not just to the Middle East but to the entire world. While the efforts of a U.S.-led coalition have weakened IS, destroying the group has proved difficult — and it has continued to inspire attacks in faraway places, from Brussels to Bangladesh.

To understand how to defeat IS once and for all, we first need to comprehend its strategy. And make no mistake: Even if the IS-associated international attacks seem random, the group's global crusade does have a strategic logic.

IS is fighting for its survival. It has neither the money nor the manpower to fight anything like a traditional war against the U.S.-led coalition and its local allies — at least not for long. What it does have is a message that resonates with certain groups — typically marginalized, disenchanted and tormented young men — within a broad range of countries in the Middle East, Europe and elsewhere. And it has become very good at tapping these sources of manpower.