The death of Anwar Al Awlaki will have a significant effect on Al Qaeda's most dangerous offshoot, Al Qaeda in the Arabian Peninsula (AQAP), which operates in Yemen and Saudi Arabia.
AQAP will now find it harder to recruit, inspire and raise money, experts said.
The demise of the US-born radical cleric will also deepen the sense of doom among other key Al Qaeda figures across the region. Their ranks have been greatly shrunk by a devastating, 18-month US drone campaign.
"Awlaki's death is extremely significant," said Abdelbari Atwan, the editor-in-chief of the pan-Arab daily al-Quds Al-Arabi, based in London.
"He was charismatic, a very good speaker in both English and Arabic, and he knew how to talk to the new generation of Al Qaeda," said Mr Atwan, the author of The Secret History of Al Qaeda.
Jason Burke, another Al Qaeda expert, agreed Al Awlaki's death was significant because he was one of the organisation's few leaders who "could bridge the gap between the West and the Middle East".
But Mr Burke cautioned there was a danger of overestimating Al Awlaki's importance because of his high profile in the West.
"His primary role was propaganda and the dissemination of ideology, not operations," the author of The September 11 Wars said. "So the impact is going to be more in the realm of those more intangible elements."
IntelCenter, a private US company that monitors jihadist organisations, said that despite Al Awlaki's death AQAP still poses a "direct threat to the US".
It added the group's leader, Nasir Al Wuhayshi, who is blamed for planning attacks on US territory, remains at large.
But Mr Al Wuhayshi will be scared. And while he is watching his back, it will be harder for him to plan terrorist attacks on the US.
Unmanned US drones have been killing Al Qaeda leaders faster than they can be replaced.
In August, a drone strike killed Atiyah Abd Al Rahman, the Al Qaeda deputy, in Pakistan's tribal areas.
A month earlier, the US defence secretary Leon Panetta proclaimed the US was "within reach of strategically defeating Al Qaeda". Washington's focus, Mr Panetta said, would be to kill or capture between 10 and 20 key members of the terrorist network in Yemen, Pakistan and Somalia.
He singled out as prime targets Osama bin Laden's uninspiring, Egyptian-born successor, Ayman Al Zawahiri - and Al Awlaki, the first US citizen to be marked for assassination by the CIA.
