Last year, the Islamic State released a training video, one of a multipart series shot in Iraq. With its scenes of foot drills, target practice, and karate chops, it would have been entirely unremarkable were it not for a short classroom scene, in which an instructor walks viewers through the ideological curriculum forced upon new recruits to the ISIS cause. As he’s shown reeling off a list of some key topics in jihadist jurisprudence, one can glimpse a thick volume resting atop each of the 20 or so schoolroom desks—a manuscript that, while few would recognize it outside of jihadist circles, is instrumental to ISIS as a theological playbook that is used to justify the group’s most abhorrent acts.

Recently, a Pentagon spokesman confirmed that the obscure author of this book, Abu Abdullah al-Muhajir, had been killed in a corner of northeast Syria by an American strike. Notably, at the time of his death, he was not affiliated to ISIS but, rather, its chief ideological rival in Syria, Jabhat Fateh al-Sham (formerly Jabhat al-Nusra), a group whose orbit he entered into sometime in the last few years.

Mystery and intrigue shroud the life of Muhajir, a man who has a rich aural lineage (literally days’ worth of online recordings) but who only appeared on camera for the first time in June of this year. However, while there is a striking paucity of open-source information about him, the Egyptian national, a veteran of the Afghan jihad and long-time al-Qaeda associate, had a massive impact upon the development of jihadist thought in the last four decades. Indeed, it’s hard to overstate his importance in the context of modern Islamist terrorism—neither the Islamic State nor al-Qaeda would be where they are today without him.

To truly grasp Muhajir’s significance, one needs to go back to the beginning of the global jihadist movement, 1980s Afghanistan. It was there that Muhajir cut his extremist teeth, mingling with the likes of Osama bin Laden and Ayman al-Zawahiri—the man who replaced bin Laden at the helm of al-Qaeda following the founder’s death—fighting alongside them and, according to long-time Taliban strategist Mustafa Hamid, rallying against them too. Back then at the height of his extremism, Muhajir deemed the Taliban to be too accommodating and at one point even refused to admit bin Laden to the training camp he administered at Khaldan.

Notwithstanding his early rejectionism, though, Muhajir softened over time, eventually ending up back in al-Qaeda’s good books, soon to become one of its foremost scholarly authorities. It was during this time—around 2000—that he first substantively crossed paths with Abu Mus’ab al-Zarqawi, the founder of the Islamic State’s precursor group al-Qaeda in Iraq, and a man that remains today the single most important individual in the Islamic State’s material history.

Muhajir made quite an impression on the young Zarqawi. In a 2005 statement, for example, Zarqawi used the Egyptian’s work to legitimize his group’s indiscriminate killing of Muslim civilians in Iraq. The admiration didn’t stop there—according to the al-Qaeda in Iraq chief’s former companion Maysara al-Gharib, Zarqawi actually tried to get Muhajir to Mesopotamia to head up his newly formed Sharia Committee and, upon failing to convince him, set Muhajir’s two main books as core texts at AQI training camps (one of which, commonly known as The Jurisprudence of Blood, was the book we referred to at the beginning of this article).

For AQI, Muhajir’s work presented the theory behind its practice. He was, in a sense, the theological brains behind Zarqawi’s ultraviolent brawn, and his intellectual contributions will live well beyond his death. Ranging from ruminations on the merits of beheading, torturing, or burning prisoners to thoughts on assassination, siege warfare, and the use of biological weapons, Muhajir’s intellectual legacy will remain a crucial component of the literary corpus of ISIS—and, indeed, whatever comes after it—as a way to render practically anything permissible, provided, that is, it can be spun as beneficial to the jihad.

This has been already clear in the context of suicide tactics, which have characterized Salafi-jihadist violence for well over a decade now (despite the Koran’s clear prohibitions against killing oneself). According to Muhajir, committing suicide to kill people is not only a theologically sound act, but a commendable one, too, something to be cherished and celebrated regardless of its outcome.

Whereas other jihadists, most notoriously Zarqawi’s erstwhile mentor Abu Muhammad al-Maqdisi, did not dismiss the permissibility of suicide operations out of hand, they did express serious trepidation about “fling[ing] the door wide open” to the tactic. Muhajir, on the other hand, cast theological caution to the wind. So convincing was his argumentation that Zarqawi—who, by his own testimony, “used to believe in the impermissibility of martyrdom-seeking operations” like his mentor—cited Muhajir’s influence as the sole reason for his wholesale adoption of the tactic in Iraq in the 2000s, a period that ushered in some of the worst sectarian violence of the Iraq War. It’s with an extract from this very speech, which Zarqawi made in response to allegations raised by his former mentor, Abu Muhammad al-Maqdisi, that ISIS opens up its official edition of The Jurisprudence of Blood:

When we were released from prison and I went to Afghanistan again, I met with Shaykh Abu Abdullah al-Muhajir. A conversation ensued between us on the ruling of martyrdom-seeking operations. The Shaykh was of the view that they are permissible. I read his valuable research on the issue and listened to many of his recordings. Eventually, Allah opened my heart to his position so that not only did I come to see them as permissible, but as desirable, too. Since effecting that ideological sea-change, neither Zarqawi nor his inheritors have looked back, liberally using Muhajir’s work to normalize the use of suicide tactics in the time since, such that they have become the single most important military and terrorist method—defensive or offensive—used by the Islamic State today.

The way that Muhajir theorized it was simple—he offered up a theological fix that allows any who desire it to sidestep the Koranic injunctions against suicide. Essentially, his position boils down to the attack’s “purpose and intent.” Suicide with the intent of ending personal woe, he held, is forbidden because it implies that the person in question is willfully ignorant of God’s mercy. However, if the intent is to “support” and “uphold” the religion, the same act of killing oneself becomes something honorable.

Lots of jurisprudents made similar arguments about suicide tactics, many of them before Muhajir—the majority stating that they become permissible if the suicide attacker’s intention is correct, the operation is occurring as a last resort in a state of war, and that it has significant benefit to the Muslim community. But the Egyptian lowered the bar for what constituted “benefit.” For him, suicide attacks didn’t need war-altering results in order to be permissible; the attacker just needed to want to die for the “right” reason. It doesn’t take a genius to see how this kind of flexibility works in the favor of groups like al-Qaeda and ISIS.

Given his staggering ideological relevance to ISIS, it’s somewhat surprising that Muhajir ended up dying on the side of one of its most ardent adversaries. According to one former Jabhat al-Nusra leader, the animosity he felt toward his ideological progeny was intense, evidenced by an unsubstantiated claim that his “last wish” was to fight in the front lines against the self-proclaimed caliphate. Yet Muhajir’s tale fits a pattern; turncoat politics have long shaped Salafi-jihadism.

In any case, though, the precise circumstances of his death matter relatively little—particularly in this context, intellectual contributions can transcend group identity, especially when they have the ability to shape history.