Abu Muhammad al-Adnani the chief propagandist and strategist of ISIS, was killed on Tuesday. He was one of the organization’s most viscerally and publicly aggressive leaders.

Two years ago, the United States put a five-million-dollar bounty on the head of Abu Muhammad al-Adnani, the chief propagandist and strategist of the Islamic State, also known as ISIS or ISIL. He had used some of the most repugnant language to come out of the jihadi caliphate. In September, 2014, after the Obama Administration mobilized a coalition to conduct air strikes against ISIS, Adnani responded with a long and rambling challenge:

O Obama. O mule of the Jews. You are vile. And you will be disappointed, Obama. Is this all you were capable of doing in this campaign of yours? Is this how far America has reached of incapacity and weakness? Are America and all its allies from amongst the crusaders and atheists unable to come down to the ground? Have you not realized—oh, crusaders— that proxy wars have not availed you nor will they ever avail you? Have you not realized, O mule of the Jews, that the battle cannot be decided from the air at all? Or do you think that you are smarter than Bush, your obeyed fool, when he brought the armies of the cross and placed them under the fire of the mujahidin on the ground? No, you are more foolish than him.

The United States appears finally to have got its man—one of the most wanted targets on its terrorism list—on Tuesday. In an elegiac statement, the Islamic State announced Adnani’s “martyrdom,” while “surveying military operations” in Aleppo province. “After a journey filled with sacrifice and defense against disbelief and its party, the lion-like Abu Muhammad al Adnani al-Shami dismounted, to join the caravan of martyred leaders, the caravan of heroes who waged jihad . . . and spoke the truth aloud while the death lied in wait for them,” ISIS said.

Within hours, a Defense Department official in Washington issued a terse statement: “With regards to the reports about the death of ISIL leader Al-Adnani, I can confirm on background that earlier this morning . . . coalition forces conducted an airstrike in Al-Bab, Syria, targeting an ISIL senior leader. We are still assessing the results of the operation at this time.” Al-Bab, which is held by ISIS, is in northern Aleppo province, near the border with Turkey.

Adnani’s demise, if confirmed, would be the biggest single setback to the leadership of ISIS since the group began its blitzkrieg across the Middle East, redrawing a century-old map in its wake. The caliphate’s emir is the elusive Abu Bakr al-Baghdadi, a cleric and Islamic scholar. Adnani was his chief of operations, both inside the Islamic State and abroad.

“Besides Baghdadi, he’s the second-most central figure,” a U.S. official involved in the war against ISIS told me on Tuesday. “He’s central to military planning, central to messaging efforts. He’s the voice of ISIL, and he has been the one advocating for all these horrific attacks in Iraq and Syria and around the world. He has been crucial to their efforts. If it’s true, it’s a significant setback to them.” Adnani gained fame for churning out slick videos of ISIS beheading Western hostages and gunning down local opponents in mass executions, with the black ISIS flag flying in the background. His bloodthirsty recruiting tactics attracted thousands of foreign fighters, from five continents.

Hassan Hassan, the author of the Times best-seller “ISIS: Inside the Army of Terror,” described Adnani as one of a small handful of leaders left from among the organization’s founding fathers. “This means that the transition to the second and third tiers of the group is already well under way. And this could affect the direction of the organization and how it operates,” Hassan told me. “Those leaders who grew up within this organization are more attuned to the local dynamics, so the decapitation of such leaders could, in fact, inject a new life into the group. That said, the Islamic State is already shaped and well defined by those founding fathers, strategically and ideologically, so these new leaders have little wiggle room to make a change, but this is more possible than before.”

Adnani, who was in his late thirties, had long been an American nemesis, dating back to 2003. His real name was Taha Sobhi Falaha, but he used at least half a dozen other names during his life. He was one of the first foreign fighters to target the U.S.-led coalition in Iraq. He was an early adherent of Al Qaeda in Iraq. The United States actually had Adnani in custody for more than five years, according to a United Nations biography that accompanied the imposition of international sanctions on him, in 2014. He was nabbed in 2005, under one of many aliases, and not released until 2010. He then returned to Syria and became part of ISIS. “He is one of the most influential emirs of ISIL,” the U.N. biography said.

Adnani often appealed to sympathizers abroad, especially to lone wolves operating in the West, to launch their own initiatives. “If you are not able to find an I.E.D. or a bullet, then single out the disbelieving American, Frenchman, or any of their allies. Smash his head with a rock, or slaughter him with a knife, or run him over with your car, or throw him down from a high place, or choke him, or poison him,” he urged.

He was widely believed to have had a hand, directly or indirectly, in various bombings and attacks in Europe and the United States, including the San Bernardino shootings, in December. ISIS sympathizers are also linked to the attacks this year at the Orlando night club Pulse, the Bastille Day celebration in Nice, and a host of bloody suicide bombings in the Middle East during Ramadan, the holiest month on the Muslim calendar. “Adnani was arguably the most viscerally aggressive ISIS leader in the public eye, and it was his repeated calls for attacks by supporters and members in the West that have been so successful in recent months,” Charles Lister, a senior fellow at the Middle East Institute, in Washington, told me. “Without his explosive voice, ISIS may find it hard to inspire the intense levels of violence that it has managed to inspire of late.”

But Adnani’s death almost certainly will not cripple ISIS, any more than the raid that killed Osama bin Laden crippled Al Qaeda. ISIS has already pledged revenge for Adnani’s death. “We give glad tidings to the filthy cowards in the sect of disbelief, and the carrier of the banner of the Cross in it, with what will deprive them of sleep in their beds,” it warned. “The blood of the sheikhs will only make it more firm on the path of jihad and determination to take revenge and assault.”

The United States did not seem fazed by the threat, at least publicly. Late Tuesday, the Pentagon press secretary, Peter Cook, confirmed that it had fired a precision strike intended for Adnani. He pledged that the U.S. military will continue to “relentlessly target ISIL leaders and external plotters” to defend the United States and its allies as they “gather momentum in destroying ISIL’s parent tumor in Iraq and Syria and combat its metastases around the world.” The showdown between the world’s mightiest power and its most virulent adversary still has a long way to go.