AMOMENTOUS event is about to unfold in the Middle East. Twenty months after Israeli Prime Minister Ariel Sharon stunned his countrymen and the world by urging the evacuation of Jewish settlers and troops from the Gaza Strip, Israeli forces are about to execute his plan. Starting tomorrow, more than 40,000 Israeli soldiers and police will begin clearing some 8,500 settlers from all 21 Gaza settlements, plus four more small outposts in the northern West Bank. The withdrawal itself, and how it proceeds over the coming weeks and months, is a crucial test for Israelis, Palestinians and the Bush administration, containing the seeds of hope for a durable peace but also risks and pitfalls for both sides.

Mr. Sharon, by dint of his formidable will, has driven his policy from conception to reality, surmounting the emotional opposition of his own Likud Party members as well as that of Israeli extremists and militants, some of whom are likely to do their utmost to sabotage the evacuation. Many or most Israelis, even some in the majority that supports Mr. Sharon's policy, believe he is risking the country's security; certainly, given the passions his move has provoked, he is risking his own life. It is therefore crucial that the Bush administration, which until recently paid too little heed to the Israeli-Palestinian conflict, focus on it now, and on the need to support Mr. Sharon both through this withdrawal and on to the next phase of Mideast peacemaking.

Mr. Sharon's challenge is to manage the fissures his policy has aggravated in Israeli society while simultaneously exercising restraint in the face of Palestinian provocations if they occur. Certainly, he will have his hands full with the evacuation himself, not least from an estimated 2,700 Israeli militants, mostly settlers from the West Bank, who have infiltrated Gaza settlements in recent weeks in hopes of subverting the Israeli withdrawal. But even after the Israelis have left, with luck by early September, Mr. Sharon will have no small influence over events in Gaza, including the political fortunes of the moderate but weak Palestinian Authority president, Mahmoud Abbas. If isolated Palestinian attacks prompt Mr. Sharon to order that Israeli forces sweep and pulverize Gaza, he will only undercut Mr. Abbas and strengthen the hand of militant Palestinian groups, such as Hamas, that are vying for control of Palestinian territories.

Mr. Abbas, for his part, has urged Palestinians to stay calm and orderly as the Israeli withdrawal proceeds. "Let them go," he said in a televised address last week. "Let us allow them to leave." But the test for him will be matching his words with deeds. In the absence of deep popular support in the seven months since he succeeded Yasser Arafat, Mr. Abbas has shied away from confronting Palestinian militants. The time for accommodation is finished. If Gaza is swept by riots, or if Hamas is allowed to rain rockets and AK-47 fire on Israeli soldiers and civilians as they depart or after, Israel and the world will rightly doubt not only Mr. Abbas's leadership but also the Palestinians' readiness to take control of their own state alongside Israel.

Washington and its allies must nudge the two sides into coordinating post-withdrawal arrangements on security, border crossings and the movements of goods and people. In this regard, James D. Wolfensohn, the international envoy who is managing the withdrawal, has been a major constructive force, even donating $500,000 of his own money to a private fund that is buying Israeli greenhouses in Gaza in order to transfer them to the Palestinians.

The stakes are high for Washington, which is presented with an opportunity to demonstrate that it can play an evenhanded, constructive role in the Mideast. For the Bush administration, the key objectives are twofold: to press the Palestinians to uphold their security commitments and to ensure that the Israeli withdrawal is the start of fresh momentum toward peace, not the end of it. If, as many believe, Mr. Sharon's intention is to quit Gaza, then put all talks with the Palestinians on ice indefinitely, then "Gaza first" will become "Gaza last," and the Bush administration will have missed a rare chance to break the impasse in the region's most durable conflict.