Prime Minister Ariel Sharon and President Mahmoud Abbas have created a promising moment in the 58-year-old war known as the Israeli-Palestinian conflict. And both are paying for such audacity, which will open the way for them to make even bigger sacrifices.

The Israeli withdrawal from the Gaza Strip that begins this week is a necessary step toward peace. So is the construction of effective interim security barriers between the Palestinian territories and Israel. A tacit understanding between Sharon and Abbas to push forward with both measures simul- taneously has opened an immediate window of opportunity.

But these steps alone cannot transform the conflict. Israelis and Palestinians need to accept at this point that such a transformation will come only through visionary political leadership by the protagonists -- and focused support from abroad -- for the tougher challenges that come next.

Sharon and Abbas present contrasting images in leadership as they navigate the treacherous currents that the Gaza withdrawal sets in motion. Sharon operates as a strong commander who will bull his way through the opposition to withdrawal but whose ultimate intentions toward the Palestinians remain unclear -- perhaps even to himself. Abbas registers as a sincere, well-motivated politician who lacks the means and will to deliver on his promises of moderation.

The leadership of their own parties is splintering behind both men as they push ahead on Gaza. Sharon is attacked from the right by Binyamin Netanyahu and the settlers Netanyahu cultivates. Abbas is undermined from the left, in subtle and not-so-subtle ways, by Prime Minister Ahmed Qureia, who runs with the Palestinian rejectionists.

The splintering reflects the chal- lengers' ambitions and their sense that Gaza withdrawal could aggravate, rather than lessen, the underlying stalemate that began with the 1947-48 Arab-Israeli war. That is not to say that Netanyahu and Qureia necessarily want that outcome. But it is what each expects.

How to prove them wrong? By using the Gaza withdrawal and the continuing construction of the security fences around Gaza and the West Bank to transform rather than freeze the conflict. The obligations fall mutually, if not equally, to both Israelis and Palestinians as the second, terrorism-driven intifada burns itself out.

Sharon's politically brave decision to remove Israeli settlements from Gaza points the way to what is needed next. If a peaceful, cooperative evacuation from Gaza -- which Abbas has endorsed -- occurs, then Israel should quickly commit itself to removing its settlements from the West Bank territory that it will return to Arab inhabitants. Half a century of turmoil has shown that this step is a condition for peace, and Sharon has set the stage for it.

The commitment to transformation rather than immobility must also be shown through the exact route and function of Israel's security "fence" -- in fact a collection of buffer zones with electronic sensors, intimidating walls, futuristic "terminals" to allow the controlled transit of goods and people, prosaic wire barriers and other devices.

The fence must be an instrument of intermediation and of buying time for reconciliation -- not an instrument of absolute control by one people over another.

The immediate need from the Palestinians -- in addition to the cessation of the campaign of terrorist attacks on Israelis -- is to treat the Gaza withdrawal as an act of peace and not of armed liberation. Creating a false narrative of having "defeated" the Israelis by the force of arms will perpetuate the conflict and Israeli occupation, direct or indirect.

Why dream this could ever happen? Because enlightened leadership has created miracles before, in general terms (South Africa is an outstanding example) and specifically in the failed but still relevant negotiations between Israelis and Palestinians in 2000.

Those negotiations produced an outline of a settlement, known in peace-process jargon as the Clinton parameters, that returns 92 to 95 percent of the West Bank to the Palestinians. It also evacuates most Israeli settlements and provides a carefully drawn compromise on the right of return of Palestinian refugees. The matrix of a durable agreement already exists.

The guiding principle to such an agreement must recognize that the Israeli-Palestinian conflict is rooted in a collision of two just causes: Jewish victims of the Holocaust seeking a nation to guarantee their survival did dispossess an indigenous Arab Palestinian people, whose needs were then neglected or suppressed -- most of all by the Arab regimes that claimed to champion the Palestinians.

Gaza withdrawal and dissoluble fences present an opportunity to remake the conflict, which for too long has been dominated by a contest of wrongs rather than a collision of rights.

jimhoagland@washpost.com