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.