AS PART OF HIS new pro-democracy policy for the Middle East, President Obama last week called on a longtime U.S. ally, Yemen’s President Ali Abdullah Saleh, to step down. For a few hours on Sunday, it looked as though the strongman might comply. Then an armed mob of the president’s followers surrounded an embassy where the U.S. ambassador had gathered with Arab and European envoys. After being trapped for several hours, the ambassadors escaped to the presidential palace — only to watch as Mr. Saleh reneged on a deal that would end his 32 years in power. On Monday major fighting broke out for the first time between pro- and anti-Saleh forces in the capital, Sanaa.

The crisis presents a quick test of whether Mr. Obama will follow through on his commitment to back democratic transitions across the Middle East — including in states where autocratic rulers have been American allies. Mr. Saleh has been a well-financed if unsteady U.S. partner against al-Qaeda, which has developed a base of operations in Yemen that may be more dangerous than that in Pakistan. But he doomed his regime in March by ordering his security forces to open fire on peaceful pro-democracy protesters. Major army units and tribal leaders defected to the opposition, leaving Mr. Saleh with little control over the country — and raising the risk of unhindered operations by al-Qaeda.

Since early April the Gulf Cooperation Council, a group of emirates led by Saudi Arabia, has patiently negotiated a deal under which Mr. Saleh would step down in 30 days in exchange for immunity from prosecution. New democratic elections for president would then be held. Three times Mr. Saleh has agreed to the deal; three times he has reversed himself. His latest excuse was the failure of opposition leaders, who had signed the accord on Saturday, to do so in his presence. This absurd maneuver, and provocations by his security forces, have now brought Yemen to the brink of civil war.

The United States has a vital interest in restoring order to Yemen. But as so often has been the case during the Arab Spring, the administration’s first response to the new crisis was mild. Secretary of State Hillary Rodham Clinton issued a statement saying “the United States is deeply disappointed.” Administration officials and U.S. allies were reported to be reviewing military and economic support for Yemen on Monday in a search for leverage against Mr. Saleh. Mr. Obama must now make clear that he is prepared to act on his words. Any remaining U.S. aid and support to Mr. Saleh and his forces should be ended, and the strongman should be told that he will be subject to sanctions and criminal prosecution unless he immediately accepts the transition agreement.

In the meantime, U.S. forces should be prepared to act independently against al-Qaeda targets in Yemen. Earlier this month, a drone strike targeted — but missed — Anwar al-Aulaqi, the al-Qaeda leader who has authored terrorist plots against the United States. While Yemen remains in chaos, such operations must continue.