The killing of Anwar al-Awlaki by a US drone is significant, unlike that of other al-Qa'ida operatives, because he was one the few effective propagandists in the group. He did not direct bomb makers, but his sermons inspired Muslims to violent action.
In 2010, his words led a 21-year-old student in Britain called Roshonara Choudhry to stab and wound a member of parliament who had supported the Iraq war. She later told police that she had decided to act, without consulting anybody, after listening to Awlaki's lectures on the internet for 100 hours. "I told no one. No one else would have understood," she said.
No other figure in al-Qa'ida had the same power with words as US-born, Yemen-based Awlaki, who could speak in fluent English. In his YouTube lectures he speaks with an easy-going and confident clarity.
A further reason for the US to be pleased by Awlaki's death is that the group to which he was affiliated, al-Qa'ida in the Arabian Peninsula (AQAP), was genuinely interested in striking at US targets. It may seem self-evident that al-Qa'ida is chiefly devoted to a holy war against non-Muslim great powers, but it is by no means the case, even if this was the aim of Osama bin Laden and Ayman al-Zawahiri.
But the most powerful al-Qa'ida franchisees, notably in Iraq and, to a lesser extent, Pakistan, have shown much more interest in attacking local targets, often Shia whom they see as heretics, than the US. Al-Qa'ida in Mesopotamia carries out elaborate multiple suicide attacks against Shia and Iraqi government targets, but seldom against US troops in Iraq and never against US and European targets outside Iraq.
Al-Qa'ida in Yemen has always been much more orientated towards assaults on US targets. On 12 October 2000, two suicide bombers steered a power boat packed with explosives into the side of the USS Cole off Aden harbour, blowing a hole in the destroyer's side and killing 17 sailors.
This targeting of the US from Yemen has continued. It was from his base in Yemen that Awlaki influenced Major Nidal Malik Hassan, the army psychiatrist who carried out the shootings at the army base at Fort Hood, Texas in which 13 people were killed in 2009.
Yemen was a good refuge for al-Qa'ida because its central government is weak and it is possible – particularly for those people with good tribal connections – to find safe havens in the country.
The Yemeni government under President Ali Abdullah Saleh has always known more about the membership and locations of these groups than it cares to admit to the US. His Yemeni critics accuse him of manipulating and exaggerating the threat from AQAP, whom a Yemeni official says numbers only 300, to extract weapons and funding from the US.
It was always in the interests of President Saleh in Sanaa to be America's local ally against al-Qa'ida, but without being so successful that the group was eliminated, bringing an end to the Yemeni government's leverage over Washington. Pakistan played very much the same game after 9/11, handing over al-Qa'ida militants, but being much more protective of Taliban leaders.
Will al-Qa'ida now be fatally weakened by the death of al-Awlaki?
Probably less than would be expected, mainly because al-Qa'ida was never the structured guerrilla army that was portrayed by the Pentagon, CIA and the media. It has by now a highly depleted leadership cadre, but its significance in recent years has mostly been symbolic.
Suicide bomb tactics and Jihadi ideology, once identified with al-Qa'ida, has now become a feature of other Islamic fundamentalist movements.
Al Qa'ida cannot do much, but then it does not have to. Awlaki was accused by Yemeni officials of having contacts with Umar Farouk Abdulmutallab, the Nigerian engineering student, who tried to detonate a bomb sewn into his underpants in a plane landing in Detroit in 2009. But, although the attempt was a failure, it caused almost as much disruption and media attention as if it had succeeded.
The group is therefore unlikely to wither away entirely. Too many people owe their jobs and their budgets to pursuing it. But could it become irrelevant in the changed political situation following the Arab Awakening?
There was much optimism about this earlier in the year as regimes toppled in Egypt and Tunisia. Islamists in North Africa now had many other options for action than taking up arms and, if they do take arms, it may not be against the US and its friends. The indestructibility of dictators such as President Hosni Mubarak no longer seemed to be guaranteed by Washington and there is less reason to strike at US targets than 10 years earlier.
This argument is partly true. There is less and less incentive for local insurgencies to adopt the al-Qa'ida franchise and many reasons for them not to invite US hostility. Al-Qa'ida's former allies in Libya are now part of the Nato-backed government in Tripoli. Al-Qa'ida itself is far less popular than it once was in countries such as Jordan after it started making attacks within the country against Jordanians.
At the same time, President Obama's full-blown support for Israel and the US military presence in Afghanistan means that the anti-American motivation of Jihadi groups will not disappear.
Al-Qa'ida itself may become weaker and weaker, but its tactics and also its aims may start to be adopted by other, less identifiable, groups.