He died Oct. 26 in northwest Syria, during a raid conducted by Special Operations forces, President Trump said in a Sunday morning news conference at the White House. Mr. Baghdadi was 48, and had run into a “dead-end tunnel” before he “ignited his vest,” killing himself and three of his children, Trump said.

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“Baghdadi was vicious and violent, and he died in a vicious and violent way — as a coward, running and crying,” the president added.

The man at the helm of the Islamic State was a shadowy presence, appearing in public only a handful of times and rarely allowing his own voice to be heard, even as the caliphate was beaten back and finally destroyed. During his tenure, the Islamic State would come to mirror its leader: a messianic figure drawn to the harshest interpretations of Islamic texts and seized with the conviction that all dissenters should be put to death.

And yet, despite the group’s extremist views and vicious tactics, Mr. Baghdadi maintained a canny pragmatism as leader, melding a fractious mix of radical jihadists and former Iraqi Baathists and army officers into an effective military force. It was this combination of extremist ideology and practical military experience that enabled the group to seize and hold territory that would form the basis of a declared Islamic caliphate.

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“He was the guy who could build bridges between the foreign fighters and local Iraqis,” said William McCants, a scholar of militant Islam and author of “The ISIS Apocalypse,” a 2015 history of the Islamic State, which is also called ISIS. “His ability to move between these two factions helped his rise to become caliph and then allowed him to stay on top.”

Although rivals and foes would initially dismiss him as a mere spiritual leader and figurehead, Mr. Baghdadi “proved himself to be a capable and cagey leader, politically,” McCants said.

Mr. Baghdadi’s public profile diminished considerably after 2015, amid frequent, unverified reports that he had been wounded or even killed in an airstrike. His last confirmed statement was an audio address, posted to jihadist websites in November 2016, offering encouragement to followers after the Iraqi government began its campaign to retake Mosul.

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Conservative academic

The man who would become the founding leader of the world’s most brutal terrorist group spent his early adult years as an obscure academic, aiming for a quiet life as a professor of Islamic law. But the U.S. invasion of Iraq in 2003 upended his plans and launched him on a course toward insurgency, prison and violent jihad.

He was born Ibrahim Awwad Ibrahim al-Badri in the central Iraqi city of Samarra on July 28, 1971. He grew up in a devout Sunni Muslim family that included several clerics and claimed to descend from the prophet Muhammad. That assertion later proved vital to Mr. Baghdadi’s efforts to anoint himself as “caliph,” or leader of the Islamic caliphate.

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From his teens, he was fascinated with Islamic history and the intricacies of Islamic law. Acquaintances would remember him as a shy, nearsighted youth who liked soccer but preferred to spend his free time at the local mosque.

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“He always had religious or other books attached on the back of his bike,” Tariq Hameed, an acquaintance from the same lower-middle-class neighborhood, told a Newsweek interviewer in 2014. The young Ibrahim disdained the Western clothes popular with Samarra’s young men, preferring the traditional prayer cap, beard and white dishdasha robe of the religiously devout, neighbors said.

He graduated from the University of Baghdad in 1996 and received a master’s degree in Koranic recitation from the Saddam University for Islamic Studies in 1999. Immersing himself in the arcane world of 7th-century religious codes, he grew increasingly conservative. Acquaintances remembered how the college-age Mr. Baghdadi took offense at the sight of men and women dancing in the same room during wedding celebrations.

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By 2003, at age 31, he was well on his way to a doctorate and a shot at a full professorship. But after U.S. troops invaded Iraq that year, he signed up with a local resistance movement, explaining afterward that he did so as a religious duty. It would take four more years, until 2007, before he returned to school to defend his dissertation, also in Koranic recitation.

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He was arrested in the Iraqi city of Fallujah in 2004 and, in a fateful turn, landed at the notorious and now-defunct Camp Bucca prison. The vast, U.S.-run detention facility warehoused nearly 26,000 Iraqi men at a time in communal tents, and U.S. military officials later acknowledged that it served at times as a recruitment and training center for jihadists.

“Extremists mingled with moderates in every compound,” Vasilios Tasikas, who served at the time as a Coast Guard lieutenant commander in charge of legal operations at the prison, wrote in a 2009 essay in the Military Review. Over time, he wrote, the mixing of hardened jihadists and Iraqi civilians “fueled the insurgency inside the wire.”

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Mr. Baghdadi, as he began to call himself, forged a number of important alliances in the camp, befriending several members of the terrorist network run by Abu Musab al-Zarqawi, the Jordanian who founded al-Qaeda in Iraq, the notoriously brutal group known for beheading hostages and attacking relief organizations and Shiite mosques and schools.

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After he was released by the prison’s U.S. overseers in late 2004, Mr. Baghdadi became a Zarqawi disciple, gradually rising through the organization to become a religious instructor and adviser to local terrorist cells in Iraq’s Anbar province.

In 2006, U.S. forces struck a series of major blows against al-Qaeda in Iraq, killing Zarqawi and wiping out entire branches of the group’s senior leadership chart. By 2008, U.S. intelligence officials viewed the organization as all but defeated. Mr. Baghdadi managed to avoid capture and, by 2010, he had risen to the No. 3 position — senior spiritual adviser.

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When Iraqi and U.S. troops killed the group’s top two commanders that year, Mr. Baghdadi was thrust suddenly into the No. 1 leadership position as the head of a rebranded, but badly weakened, organization that had begun to call itself the Islamic State of Iraq.

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Mr. Baghdadi’s backers in his new role included a number of former officers of Saddam Hussein’s vanquished army — Sunni colonels and majors who had initially allied themselves with Zarqawi had moved to assert Iraqi control over the group after his death. To them, Mr. Baghdadi might have seemed an ideal figurehead: an Iraqi religious scholar with ancestral lineage to the prophet Muhammad, perfect credentials for a future head of a restored Islamic state, or caliphate, a kind of theocratic Muslim empire that has not existed since the time of the Ottomans.

In reality, even they understood that the “state” was a fiction.

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“Where is this Islamic State of Iraq that you’re talking about?” one of the leader’s wives complained, according to documents from a 2010 Iraqi court case. “We’re living in a desert!”

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Declaring the caliphate

Events elsewhere in the Middle East provided Mr. Baghdadi’s group with an unexpected opportunity. Beginning in late 2010 and continuing through 2011, the Arab Spring movement toppled leaders and sparked a succession of conflicts, including what became the vicious, sectarian-tinged civil war in Syria. For Mr. Baghdadi, the war on Iraq’s western border offered the very things his group needed most: a new cause, and a fresh and nearly boundless source of recruits and arms.

Mr. Baghdadi dispatched trusted followers to Syria in late 2011 to form an Islamist rebel group called Jabhat al-Nusra. But in 2013, when the new offshoot proved difficult to control, he plunged into the war himself.

He dispatched scores of fighters into Syria and rebranded his organization yet again, calling it the Islamic State of Iraq and al-Sham, the latter term being the ancient name for the region that encompasses Syria and most of the Levant. Popularly the group would be known by its Arabic acronym, Daesh, or in English, ISIS or ISIL. Later the name would be shortened to simply the Islamic State.

Feuding would continue for years between Mr. Baghdadi’s men and Jabhat al-Nusra, which eventually aligned with al-Qaeda. The Islamic State emerged as the most effective fighting force on the rebel side, capturing the eastern city of Raqqa and challenging other rebel groups for dominance across a swath of villages along the Euphrates River.

There, Mr. Baghdadi imposed strict Islamic law on local inhabitants, enforcing the rules with public floggings, amputations and executions, while his fighters consolidated their holdings and prepared for future expansion across Syria and Iraq.

Mr. Baghdadi still was in command a year later when the Islamic State’s forces roared into Iraq in late spring 2014, routing poorly led, demoralized Iraqi army divisions in a string of stunning defeats across the west and north.

By that June, his jihadist army controlled a third of Iraq’s territory, including Mosul, Iraq’s second-largest city. His holdings included not just real estate, but also oil fields, military bases, universities and hundreds of millions of dollars in cash. Almost overnight, the Islamic State was transformed into the wealthiest and best-armed terrorist group of all time.

A triumphant Mr. Baghdadi marched into one of Mosul’s oldest mosques to declare the start of a new Islamic caliphate, with himself as leader. Allowing cameras to record his movements, the bearded, slightly pudgy cleric climbed to the mosque’s minbar, or pulpit, to congratulate his followers on the start of what he described as a new chapter in human history.

“Know that today you are the defenders of the religion and the guards of the land of Islam,” he said.

Evoking the prophet

Mr. Baghdadi’s victories would not go unchallenged. Backed by U.S. air power, Iraqi troops would halt the Islamic State’s advance and reclaim several smaller towns and villages.

Meanwhile, Mr. Baghdadi would soon face other determined enemies, as his extravagantly staged mass executions and beheadings of Western captives evoked fear and resolve in Europe and North America. Terrorist attacks on Paris and Brussels deepened a growing international commitment to drive the Islamic State from its stronghold, depriving it of a sanctuary and a central element of its propaganda.

A new U.S. military campaign quickly drew supporters from dozens of other countries. Iraqi troops, Kurdish fighters and tribal militias slowly began liberating Iraqi and Syrian cities as U.S. drones picked off a steady succession of Islamic State leaders.

U.S.-backed Iraqi forces retook the group’s Iraqi capital of Mosul in a campaign that ran from 2015 to early 2017. The Islamic state’s Syrian capital, Raqqa, was liberated in 2017, and the caliphate’s final outposts in the country were destroyed months later.

Mr. Baghdadi remained an elusive foe, all but disappearing from public view except for a rare written statement posted to social media sites.

Likewise, he kept his family and social life carefully hidden. He married at least twice, and possibly three times, and had at least six children.

Later, former hostages would reveal that Mr. Baghdadi also kept a number of personal sex slaves during his years as the Islamic State’s leader, including slain American hostage Kayla Mueller and a number of captured Yazidi women. U.S. officials corroborated the accounts.

No such details were acknowledged by the terrorist leader himself. Indeed, unlike his charismatic predecessor Zarqawi, whose gun-toting image remains an icon within the jihadist community, Mr. Baghdadi sought to shift the focus to the caliphate itself, the mythical Islamic utopia he believed he was creating.

The goal was to impose Islamic rule without borders, and the way to achieve that was to act boldly and ruthlessly: Raise the caliphate’s ancient banner, he would say, and the world’s devout Muslims will line up behind you.

“For Baghdadi, the state-building project was a means to an end,” said Jacob Olidort, an expert on Islamist groups at the Washington Institute for Near East Policy. “The caliphate is a claim to defining and leading the Islamic community, not just in a political sense but in a definitional sense. It is the key driver.”

At least for a brief moment in 2014, Mr. Baghdadi appeared convinced that he had already achieved that goal. In his speech announcing the founding of the caliphate from Mosul, the publicity-shy Mr. Baghdadi made a very public claim as the living heir to the line of great warrior-emirs from Islam’s early history.

In his dress and movements, he symbolically evoked the prophet himself, from his black turban and robe to the traditional miswak, a carved wooden teeth-cleaning stick that Muhammad was said to have favored.

Then he called on his followers to join him in conquest.

“Prepare your arms, and supply yourselves with piety. Persevere in reciting the Koran with comprehension of its meanings and practice of its teachings,” Mr. Baghdadi said. “This is my advice to you. If you hold to it, you will conquer Rome and own the world.”