ON FRIDAY, hundreds of thousands of Egyptians did something that the Obama administration, and many others in Washington, believed they would never do: They rose up against their government, demanding an end to President Hosni Mubarak's autocracy. They overwhelmed the security forces that Mr. Mubarak deployed in an attempt to crush them; they defied a nighttime curfew even after Army units were deployed. They burned the headquarters of the ruling party in Cairo and in several other cities. By nightfall, it seemed clear that only two events could end their revolution: a massive use of force by the Army or Mr. Mubarak's yielding of power.

The United States should be using all of its influence - including the more than $1 billion in aid it supplies annually to the Egyptian military - to ensure the latter outcome. Yet, as so often has happened during the Arab uprising of the past several weeks, the Obama administration on Friday appeared to be behind events. It called for an end to the violence against demonstrators and for a lifting of the regime's shutdown of the Internet and other communications. Encouragingly, the White House press secretary said that the administration "will review our assistance posture based on events that take place in the coming days."

But U.S. statements assumed that the 30-year-long rule of the 82-year-old Mr. Mubarak would continue. After speaking to Mr. Mubarak, President Obama said Friday night that he would continue to work with the Egyptian president; he did not mention elections. Instead, in an apparent attempt to straddle the two sides, the administration suggested that the solution to the crisis would come through "engagement" between the regime and the protesters.

"We're encouraging the government . . . to try to engage in a discussion as to what the legitimate claims being made are, if they are, and to try to work them out," Vice President Biden said in a Thursday night interview on PBS, adding that he would not call Mr. Mubarak a dictator and did not think he should step down.

This view is very likely to prove as unrealistic as the administration's previous conviction that Mr. Mubarak's regime was, as Secretary of State Hillary Rodham Clinton put it on Tuesday, "stable" and "responding to the legitimate needs and interests of the Egyptian people." In fact, it is far-fetched to suppose that the aging strongman - whom the vast majority of Egyptians regard as a dictator - will agree to a serious dialogue with his opponents, much less adopt reforms he has rejected for decades.

In an address on Egyptian television early Saturday, Mr. Mubarak sounded unyielding, warning of "chaos" and portraying the country's grievances as mainly economic. His only concession was the dismissal of his cabinet - a step that will not defuse the demands for his own departure.

It's dangerous to assume that the energized and enraged Egyptian populace will be induced to stand down by any promises Mr. Mubarak might make. To question, as Mr. Biden did, whether the protesters' demands are "legitimate" is particularly obtuse. In fact, the leaders of the uprising, including former U.N. nuclear official Mohamed ElBaradei, have set forward a moderate and democratic platform. They seek the lifting of a hated emergency law that outlaws even peaceful political assembly; the right to freely organize political parties; and changes to the constitution to allow free democratic elections. Their platform could transform Egypt, and the Middle East, for the better. But the precondition for change is Mr. Mubarak's departure from office.

Rather than calling on an intransigent ruler to implement "reforms," the administration should be attempting to prepare for the peaceful implementation of the opposition platform. It should be reaching out to Mr. ElBaradei - who Friday night was reported to be under house arrest - and other mainstream opposition leaders. And it should be telling the Egyptian army, with no qualification, that the violent suppression of the uprising will rupture its relationship with the United States.