WASHINGTON — For more than a decade, Al Qaeda’s affiliate in Yemen has been one of the most dangerous terrorist organizations on the planet. The group spent years inventing explosives that are difficult to detect, including trying to disguise bombs in devices like cellphones. It has tried at least three times to blow up American airliners, without success.
But the White House’s announcement last week that the United States had killed the group’s leader, Qassim al-Rimi — confirming what The New York Times first reported several days earlier — was the latest in a string of setbacks over the past few years that have damaged the group’s ability to orchestrate or carry out operations against the West, American and European counterterrorism specialists say.
A flurry of American drone strikes in Yemen in recent years has now killed two successive leaders of the group as well as Ibrahim Hassan al-Asiri, the affiliate’s notorious bomb maker. Clashes with rival Islamic State and Houthi rebel fighters in Yemen have also weakened the group, whose full name is Al Qaeda in the Arabian Peninsula. And its once formidable jihadi news media presence has been far surpassed by the Islamic State’s.
“AQAP doesn’t seem like the beast it once was,” Edmund Fitton-Brown, a former British ambassador to Yemen who is now a top United Nations counterterrorism official, told a think-tank audience in Washington last week.
As the Yemen branch reels from these body blows, other Qaeda affiliates around the world are elbowing their way to prominence. The Shabab, an East African terrorist group affiliated with Al Qaeda, has ramped up attacks in Somalia in recent years, drawing increasing fire from American missile strikes. The group last month assaulted a Kenyan military base housing United States troops, killing three Americans.
American counterterrorism officials have voiced increased alarm about a Qaeda affiliate in Syria, Hurras al-Din, that they say is plotting attacks against the West by exploiting the chaotic security situation in the country’s northwest and the protection inadvertently afforded by Russian air defenses shielding Syrian government forces allied with Moscow.
Al Qaeda in the Arabian Peninsula has dogged President Trump since his first days in office in 2017, when the president authorized an ill-fated raid on Mr. al-Rimi’s hide-out in Yemen that left dead one member of the Navy’s elite SEAL Team 6, Chief Petty Officer William Owens.
Even though the group has been weakened, intelligence and counterterrorism officials warn that the organization remains dangerous.
The group claimed last week in an audio recording by Mr. al-Rimi — made before his death — that it had directed a Saudi military officer to carry out the shooting at a United States military base in Florida in December that killed three sailors and wounded eight people.
The group offered no evidence that it had trained the gunman, Second Lt. Mohammed Saeed Alshamrani, but it produced a copy of his will as well as correspondence that indicated that he had been in contact with Al Qaeda. Experts said those elements made the claim plausible.
At a news conference last month, the F.B.I. deputy director, David Bowdich, said that while Lieutenant Alshamrani did not appear to be motivated by one specific terrorist group, his social media comments echoed those of Anwar al-Awlaki, the radical Yemeni-American cleric and senior leader of Al Qaeda in the Arabian Peninsula who was killed in a drone strike in 2011.
“AQAP remains a significant, lingering concern, in part because the group was so fixated on aviation as a target, and the kind of bomb-making expertise that the group developed was unlikely to have been contained to just a few select individuals,” said Nicholas J. Rasmussen, a former director of the National Counterterrorism Center.
“That kind of knowledge and expertise is of course shareable and transferable — not just within AQAP but beyond to other Al Qaeda-affiliated terrorist groups and other like-minded terrorists,” said Mr. Rasmussen, who is now the acting executive director of the McCain Institute for International Leadership, a think tank.
Bill Roggio, the editor of The Long War Journal, a website run by the Foundation for Defense of Democracies that tracks military strikes against militant groups, noted that AQAP had been counted out before only to rebound through resilience, patience and a commitment to its cause.
Still, the Yemen affiliate has suffered several major blows in recent years. Nasser al-Wuhayshi — the first emir of AQAP, who was widely considered to be a likely successor to Ayman al-Zawahri, the overall leader of Al Qaeda — was killed in an American drone strike in 2015.
Four years later, Mr. Trump announced that Mr. al-Asiri, the bomb maker, had died in a drone strike in Yemen in 2017. It was Mr. al-Asiri who sewed a bomb into the underpants of Umar Farouk Abdulmutallab, who tried to detonate the device on an American airliner approaching Detroit on Christmas Day in 2009. The bomb fizzled harmlessly, and Mr. Abdulmutallab was arrested and imprisoned.
Mr. al-Asiri also designed explosive devices disguised as printer cartridges intended to blow up cargo planes over the United States in 2010, and another device meant to bring down a passenger plane in 2012. Both plots were thwarted.
And then last week, Mr. Trump confirmed in a statement that Mr. al-Rimi had been killed. The statement offered few details, but The Times reported that the C.I.A. carried out the airstrike in late January using remotely piloted drones after months of tracking him.
The White House statement noted that Mr. al-Rimi’s death degraded not just Al Qaeda’s activities in Yemen but “the global Al Qaeda movement.” Mr. al-Rimi played an important senior coordinating role with other Qaeda affiliates, avoiding the need to consult on every matter with Mr. Zawahri from his hide-out, most likely along the Pakistan-Afghanistan border.
Analysts say several hardened jihadists are among those to succeed Mr. al-Rimi as the group’s next leader.
A measure of the United States’ success against AQAP is evident in the number of American military drone strikes in Yemen, which dropped to eight last year from a peak of 125 in 2017, according to statistics compiled by Mr. Roggio’s organization. The strikes resulted in fewer high-priority targets and drove many surviving leaders underground, officials said.
“AQAP failed to capitalize on the turmoil in Yemen to recruit more members and expand its following,” said Colin P. Clarke, a senior fellow at The Soufan Center, a New York-based research organization.
“While the group did indeed make inroads with certain Sunni tribes, the past several years have been focused more on survival and less on growth and plotting external attacks,” Mr. Clarke said.
Without the high-level terrorist attacks against the West that the organization had become known for and that its brand was partly built around, its image suffered, and the group seemed to have become more insular, analysts said.
“It has been decimated by drone strikes, infiltrated by spies, fragmented by infighting and handicapped by a near total shutdown of its communications networks,” said Elisabeth Kendall, a Yemen scholar at the University of Oxford who visited the country in October. “But a die-hard core will always remain. The more Yemen continues to unravel with the recent re-escalation of hostilities, the easier AQAP will find it to survive and prosper again.”