THE KILLING of Anwar al-Aulaqi by a U.S. airstrike in Yemen on Friday delivered a significant blow to al Qaeda and — in spite of the cleric’s U.S. citizenship — was clearly justified, both legally and morally. Some analysts pointed out that Mr. Aulaqi was not the leader of al-Qaeda in the Arabian Peninsula or its robust operations in Yemen. But that misses the point of why he was dangerous, and why President Obama was right to place him on a target list. Considerable evidence supports the administration’s contention that Mr. Aulaqi played a direct role in attempted attacks on the United States, including the failed bombing of an airplane on Christmas Day 2009 and a plot to bring down two cargo planes with explosives placed in packages.

Perhaps more significant, Mr. Aulaqi, a charismatic teacher and fluent English speaker, was instrumental in inspiring would-be jihadists in the United States and other Western countries — who may pose the greatest threat of terrorism at this point. He exchanged up to 20 e-mails with Maj. Nidal Malik Hasan, the alleged killer of 13 people in a 2009 attack at Fort Hood, and he was cited as an inspiration by Faisal Shahzad, who tried to set off a car bomb in New York’s Times square in 2010. Among those who followed his bloodcurdling online calls for the murder of civilians were a group of Canadian Muslims arrested for allegedly plotting attacks in Toronto; five men convicted of planning an attack on New Jersey’s Fort Dix; and a British woman who stabbed a member of parliament.

In short, Mr. Aulaqi was at least as effective, and as dangerous, as any of al-Qaeda’s training camps in Pakistan, Afghanistan or Yemen. Unlike some al-Qaeda fighters in Yemen or Somalia, he indisputably posed a threat to the U.S. homeland. He was consequently a legal and justified target of American forces, acting under the international principle of self-defense as well as the 2001 congressional authorization of the use of force against al-Qaeda.

There is good reason to hope that Mr. Aulaqi’s death — which was apparently accompanied by the unintended slaying of a second U.S. citizen, who edited al-Qaeda’s online magazine — will mean fewer recruits and a reduced risk of attacks in the United States. However, it does not end the threat of al-Qaeda — and it does nothing to alleviate the rapidly deteriorating situation in Yemen.

Sadly, the Obama administration’s aggressiveness and commendable prowess in launching drone strikes and special forces raids against al-Qaeda has been matched by timidity and ineffectiveness in addressing the larger political and security problems in the region — a failure epitomized by the recent return to Sanaa of President Ali Abdullah Saleh after three months of medical treatment in Saudi Arabia. The administration failed to push through a deal for Mr. Saleh’s departure from office; now officials are saying closer intelligence cooperation with his government facilitated Friday’s drone strike. But drone strikes and tactical cooperation with a crippled autocracy will not end the threat of Islamic jihadism — even with spectacular successes like that of Friday. That will require U.S. help in forging a new and democratic political order, as well as economic modernization — in Yemen and in other failing states.