Although it could make a bad situation worse, Somali president Abdullahi Yusuf’s resignation late last month was necessary. Mr. Yusuf, who led a transitional government for four years, was steadily losing control of the country to Islamic insurgents. Though originally backed by the United Nations and the United States, he proved a poor leader who incited clan divisions and yielded to corruption.

Somalia has been struggling for two decades to achieve a semblance of order. But feuding warlords and, more recently, a rising Islamist movement known as al-Shabab have thwarted the formation of a viable central government. Conditions have grown so bad that the United Nations considers Somalia the world’s worst humanitarian crisis. More than a million have fled their homes, and thousands have crowded into refugee camps. For many, starvation looms. The fighting, along with Somali piracy, has made the delivery of aid difficult.

Now, on top of Mr. Yusuf’s own departure, the Ethiopian troops he invited in two years ago to help subdue the insurgency are poised to leave, threatening a new period of instability. Al-Shabab’s leaders insist they will fight until an Islamic government is installed. But most Somalis have been terrorized by the insurgency and do not embrace its strict practices. There is a slim hope that moderate leaders could prevail.

Somalia’s parliament is expected soon to elect a new president. Meanwhile, fighting is bound to intensify. Though it rejected Secretary of State Condoleezza Rice’s recent call for peacekeepers, the United Nations should redouble its efforts, particularly to ensure that humanitarian aid gets through. It must push hard to ensure that a peace agreement recently signed in Djibouti gets every chance to succeed. Rejected by Mr. Yusuf, who too often ruled in the interests of his own Darod clan, the pact grants new power to the rival Hawiye clan and to moderate Islamists.

The Bush administration’s initial strategy, of supporting Mr. Yusuf in order to go after al-Shabab, unfortunately backfired. The White House believes, plausibly, that al-Shabab is harboring al-Qaida-linked terrorists. But in backing the invasion by Ethiopians, it created fierce resentment among Somalis, and made it easier for al-Shabab to recruit new fighters. The Obama administration will have to take a careful look at the forces operating in Somalia, and try to steer a course that does not further al-Shabab’s cause.