In just a few short years, the Islamic State has accomplished a tremendous amount, carving out territory for itself out of valuable Middle Eastern real estate, building a quasi-state, wreaking a new level of havoc on the region, and launching far-flung and devastating terrorist attacks.

But what may be most striking about the Islamic State is more far-reaching than the atrocities of Raqqa, Dabiq, Mosul, Palmyra, or even Paris. It's that beyond the thousands and tens of thousands who have joined the Islamic State in the Levant, there are millions, perhaps even tens of millions, around the world who are inspired by ISIS's message. And as the Islamic State slowly grinds to military defeat at the hands of international coalitions in its heartland, it is those supporters, passive and active, who will write the next chapter of ISIS's story.

The Islamic State no doubt has its share of psychotics, sadists, and thugs who would be drawn at any opportunity to do mayhem. But it is incredibly naïve to believe that this can be the only explanation for the astonishing relative success of the group that Barack Obama once dismissed as a jay-vee team.

As with any enemy, the best way to defeat the Islamic State is to understand it. And to do that, the best place to start is a new book by Graeme Wood, The Way of the Strangers. This book gives us the best insight yet into what makes the Islamic State tick.

Wood, a national correspondent at The Atlantic and lecturer in political science at Yale, spent years from the streets of Cairo to London to the Philippines to Australia, interviewing supporters of the Islamic State and getting inside their heads. What results is a series of gripping, fascinating portraits. Wood's subjects have little cageyness towards him. Since everything is foreordained by Allah anyway, revealing your plans to a Western journalist won't change the outcome. Plus, Wood has the talented journalist's skill for interview and observation. He's an astute psychologist and a good writer to boot.

The book's implicit thesis, one which is both inarguably true and persistently denied by so many decision makers in the West, is that ideas have consequences. While the motives of any individual and group of people are always multifaceted and almost always include a good helping of interest-seeking and self-delusion, it is also impossible to deny that large sections of Islamic State members and supporters, from its leadership down to foot soldiers, make decisions on the basis of what they believe.

As the Islamic State keeps repeating over and over through its high-polish propaganda apparatus, it has a theology, and this theology has content, and an internal logic, that can be understood on its merits. Once this theology is understood, and once the proponents of this theology are actually listened to, and their actions watched, it becomes impossible to deny that this theology is a key cause (maybe not the cause, but a key cause) of the actions of the Islamic State, most of its leaders, and most of its supporters.

What's more — and this is the source of the willful blindness of elite policymakers and commentators towards the Islamic State — this theology does have Islamic roots. Bear in mind: This does not mean that the Islamic State is "Islam" or "true Islam," whatever those things mean. But it does mean that the Islamic State represents a version of Islam that is recognizably Islamic.

All Muslims agree on at least one thing, which is that Muslims should follow the example of the Prophet Muhammad. And the Prophet Muhammad did do many of the things that the Islamic State is most reviled for, such as waging absolute religious warfare, engaging in slavery, stoning adulterers, and so forth. Obviously, this does not mean that Islam can be boiled down to such violence. And indeed, historically Muslims have found many ways to recontextualize Muhammad's more problematic actions.

But around the world, there seems to be only two equally extremist views of the Koran. The first, held by Muslim fundamentalists, Islamophobes, and militant atheists, is that a sacred text like the Koran can have only one, literal interpretation, and that this literal interpretation and "Islam" are one and the same. The other, held by the sort of secular rationalists who decide Western policy and make commentary vis-à-vis Islamic groups, from Barack Obama to Ezra Klein to Reza Aslan, is that a sacred text is essentially content-free; texts are infinitely pliable, and believers have always found ways to interpret texts in ways that let them do what they wanted to do anyway, so it does not matter what the text says. In this latter view, the Islamic State exists because of psychosis and socioeconomic grievances, since the only thing people do with sacred texts is to take out of them what they brought in.

Both of these views are dead wrong.

So allow me to propose a third, more middle-ground view: A sacred text, like any text (think of Hamlet, or Plato's Republic, or the U.S. Constitution) admits of a range of interpretations, a menu, if you will. It's possible to interpret a text in a way that goes against its intent. But while it's technically feasible to make any text say anything you want with enough chutzpah or self-delusion, the actual content of the text itself will still present a range of options, and will make some options more plausible than others. You can interpret Hamlet as saying that the eponymous character was insane all along and the ghost of his father was a hallucination, or that he was never insane and faked insanity, or started out sane and then grew insane through the course of the play. Or you can interpret Hamlet as being about an Asian woman living in Argentina throwing a tea party. But it will be awfully hard to get traction with that last interpretation, at least compared to the far more plausible previous ones.

But here's the key thing: There is a menu of reasonable options through which to interpret the same text. There is no one right way to read Hamlet.

By the same token, the Islamic State might be picking the wrong options in the Islamic menu, but they're still picking from the Islamic menu. This is not a semantic point. It has real-world consequences. People who are drawn to the Islamic State are devout Muslims, and it is self-defeating to try to keep them out of radicalization by pompously lecturing them about how the Islamic State's ideology has no basis in Islam, when it is obvious that it does.

Wood, who ably takes the reader through the intellectual and theological genesis of the Islamic State, points out that a prominent "Letter to Baghdadi" signed by influential Islamic scholars and prominent figures, and making the case against Baghdadi's Caliphate on Islamic grounds, fails on several basic grounds of exegesis. The scholar Landau Tasseron notes that on many exegetical points, as Wood puts it, "the Islamic State's interpretations [of the Koran] are often more in line with historical and scholarly readings than those of the authors of the letter."

This is amateur hour. No doubt, many of those who would deny any link between the Islamic State and Islam do so out of an honorable desire to stem Islamophobia. But willful blindness is never a good idea, and especially when it leads those who would see the Islamic State defeated to shoot themselves in the foot.

But while the book does contain a helpful and fascinating overview of the Islamic State's theology, it is by no means a tract about "Islam" or a work of religious scholarship. It remains a book of journalism and its main appeal is the portrait of the characters therein, which always appear complex, and sometimes even likeable. Many of them are serious men who have done their research, come to genuine belief, and dedicated their lives to it. As Wood notes, radical Islamism disproportionately appeals to analytical, left-brain types (engineers are vastly over-represented among Islamic State recruits) who like worldviews with borders drawn in black and white. The book's characters show themselves at turns to be both human and repellent. As one reads, one wants less to direct a drone strike against them, and more to shake them out of their worldview around a cup of tea.

Wood keenly notes that even as many of ISIS's converts have tried to shed their original identities and subsume them into radical Islam, they still project those identities on it, as British jihadis wax lyrical about the welfare protections available under Shariah, which comes to sound like a sort of super National Health Service.

It's a great read. But more importantly, Wood's book reveals truths about ISIS that are hiding in plain sight — but that our leaders make themselves willfully ignorant of. They ought to read his book, too.