THE SITUATION in Yemen is as complex as it is dangerous. With the president in Saudi Arabia for medical treatment, a power vacuum looms, with a bewildering array of forces competing to fill it: the remains of his regime, opposition political parties, youthful pro-democracy protesters, renegade generals, tribal leaders and Islamic extremists. If the Obama administration and European and Arab allies are fumbling for a strategy, they have good reason to be. But there is at least a starting point on which all should be able to agree: Ali Abdullah Saleh should not be allowed to return to Yemen unless he definitively gives up the presidency — and maybe not even then.

Though unfortunate for the 69-year-old Mr. Saleh, the relative good news may be that his medical condition could by itself ensure his indefinite exile. His supporters initially said he was only lightly injured in an apparent bombing in a mosque last Friday, but U.S. officials later reported that he suffered extensive burns and a head injury, and that fragments of wood were embedded in his body.

That may be the only way to restrain a man who, after 33 years in power, has stubbornly clung to office even after appearing to accept, on three occasions, deals for his departure. Mr. Saleh’s last reneging, on May 22, touched off a low-grade civil war between security forces still loyal to him, some of them commanded by his sons, and tribal fighters. Islamic militants have meanwhile seized control of one town, and Yemen’s branch of al-Qaeda — which has sponsored at least two attempted attacks on the U.S. homeland in the past two years — is believed to be consolidating a base in several mountainous provinces.

Mr. Saleh has been, at best, an inconstant ally of the United States, despite extensive U.S. training and funding of his security forces. But few of the candidates to succeed him look better. The youth groups leading pro-democracy protests are attractive but disorganized; several dissident tribal leaders and generals are Islamists who have been accused of links to al-Qaeda. Saudi Arabia has considerable influence with opposition political parties and tribes. But U.S. and Saudi interests in the Middle East are diverging as the kingdom seeks to prevent the spread of Arab democracy.

The best available policy nevertheless appears to be that being pursued by the Obama administration, which is pressing for acceptance of the deal brokered this spring by the Saudi-led Gulf Cooperation Council. This would grant amnesty to Mr. Saleh and his family in exchange for his resignation and new presidential elections — which even the Saudis prefer to gun battles in determining the country’s next leader. Already dirt poor, Yemen will desperately need economic resuscitation when and if the current crisis can be overcome. That should provide the United States a means of leverage with a new regime.