Washington ? Wael Ghonim, the charismatic young Google executive who helped launch the protests in Tahrir Square, sounded the trumpet in a Twitter message: “Mission accomplished. Thanks to all the brave young Egyptians.” But the struggle for the future of Egypt is just beginning, and the next stage is one where the U.S. truly can help.

The resignation of President Hosni Mubarak on Friday was an astonishing victory for young Internet revolutionaries — and for their allies in the Muslim Brotherhood who have been battling Mubarak since before most of the protesters were born. This diffuse opposition movement must now help rewrite Egypt’s constitution and election laws, rebuild a shaken economy and restore stability after the heady 18-day binge of revolution.

The best thing that can be said about the Tahrir uprising is that it was truly “made in Egypt.” President Obama got hammered at home for not trying to dictate the outcome, but he was right in his initial instinct that America can influence events best when it does so quietly, behind the scenes.

The White House (prodded by the caviling news media) began to violate that rule as the crisis wore on, with officials making statements every few hours — inserting America in the crisis and at the same time insisting that it was a matter for Egyptians to resolve. This came across as a parody of pragmatism — a sense that Washington would accept whatever worked.

Listening to the babbling official voices in Washington dispensing advice to Egypt over the past two weeks, my strongest feeling was: Just shut up. Nobody wants to be seen capitulating to American demands — and more blatant American intervention could have corrupted this spontaneous, homegrown revolution. The right tactic was offstage pressure on Mubarak and his allies, as Obama argued.

Now America doesn’t have to be quite so reticent. The good guys have won — and they have appeared to do so largely on their own, which makes victory even sweeter. But they need help, of a kind that the United States and its allies uniquely can provide. I hope Washington will see this as a moment like the Marshall Plan in 1947 or the fall of the Berlin Wall in 1989. The Arab world is at an inflection point: If the Egyptians can make a transition to a strong, secure and prosperous democracy, it will affect the region for decades.

The first challenge is economic. Egypt has pushed economic growth to an average of about 5 percent, but that’s not good enough to achieve “escape velocity” from poverty and stagnation. With more than half of its population under the age of 25, Egypt needs growth approaching that of India or China. “Calculations indicate that growth rates of 6-7 percent on a sustainable basis are needed in order to provide jobs for new entrants and to reduce unemployment,” writes economist George T. Abed of the Institute of International Finance.

The Tahrir protests may have exhilarated Egyptian Internet moguls, but they were bad for business. Capital has been fleeing the country, and spreads on Egyptian debt have widened sharply. U.S. economic and financial support, post-crisis, can help restore confidence.

A second challenge will be the simple mechanics of democratic elections. This is an area where U.S. and European organizations have deep experience, after aiding the democratization of Eastern Europe. When the Egyptians ask for help, we should make a massive effort.

A final worry is security. We know from history how vulnerable countries can be in months following a revolutionary change. Provocateurs will try to destabilize the transition and hijack the process of change. Today’s optimism could be shattered by a wave of car bombings, say, or kidnappings of foreigners of the sort that shut down Beirut and Baghdad. Egypt is lucky to have a strong military that the protesters see as an ally.

The best safeguard against Muslim terrorists, hopefully, will be the sleeping giant that is the Muslim Brotherhood. Members of this group have become sharply critical of al-Qaida in recent years, which gives reason for hope. The Brotherhood, for better or worse, has won a seat at Egypt’s governing table. The formula for success, as the protesters keep repeating, is two words: No violence.

Benjamin Franklin famously answered a question about what had been accomplished at the Constitutional Convention of 1787, “A republic, if you can keep it.” That’s a useful American admonition for the revolutionaries of Cairo. America should do everything it can to help them keep what they have gloriously won.