For the Israelis who hated and feared him, the Palestinians who chafed at his corruption, or the Western and Arab diplomats who despaired at his mendacity and unwillingness to settle the conflict that was his life's work, Yasser Arafat's apparently imminent death is the best news to sweep a turbulent Middle East since before Sept. 11. But for his greatest nemesis, Ariel Sharon, it is a potential disaster -- one that threatens to undo what has been a smooth and highly advantageous relationship with President Bush.

Yes, the 76-year-old Sharon has the satisfaction of seeing the burial of an enemy with whom he has warred for decades. But the indefatigable Israeli hawk also has lost, at a stroke, the underpinning of a strategy that has allowed him to undo a decade of Israeli-Palestinian negotiations, block U.S. plans for a Palestinian state and unilaterally redraw Israel's borders -- all with the backing of a pliable White House.

Sharon may have threatened to kill or exile Arafat from time to time, but in fact the Palestinian warlord has been indispensable to him since January 2002, when he was caught trying to smuggle a boatload of Iranian weapons into the Gaza Strip even as U.S. envoys were brokering talks. A furious Bush soon agreed with Sharon that Arafat could no longer be a partner in any peace process. That has excused Sharon from any serious pressure to pursue Bush's "road map" for side-by-side Israeli and Palestinian states.

Why not the road map? Because, as he has candidly explained in occasional interviews with Israel's Hebrew-language press, Sharon doesn't like the look of the deal Israel probably would get. Most of its terms were negotiated four years ago by his predecessor as prime minister, Ehud Barak, and ratified by the Israeli cabinet. Among other things, they call for Palestinian sovereignty over Arab neighborhoods in Jerusalem, as well as up to 95 percent of the territory in the West Bank -- far-reaching concessions that an intransigent Arafat conveniently rejected.

Sharon knows Israel eventually must accept a Palestinian state, but he hopes for considerably better terms: annexation of at least 10 percent of the West Bank, exclusive Israeli control of Jerusalem and its suburbs, and a Palestinian territory divided into islands separated by Israeli roads and settlements. The world -- even the United States -- would never countenance such an outcome now. In fact, any "final status" negotiations would surely lead to intense pressure on Israel to swallow the deal it has already ratified. So Sharon hopes to put off a final settlement indefinitely -- and, in the meantime, create a new status quo that provides Israelis with a reasonable level of security.

Sharon's path to that status quo was his plan for the unilateral evacuation of Gaza, which he coupled to the construction of a borderlike security fence enclosing most of the West Bank land he wants. In April he proposed to Bush that, as a way of supporting the Gaza pullout, the United States ratify his fence as well as the eventual annexation by Israel of most or all of the Jewish settlements inside it. After quibbling a little over the route of the fence, and extracting a promise to shut down several isolated West Bank settlements, Bush agreed. Crowed Sharon in one of his Hebrew-language interviews: "A situation has been created in which it is possible to do the things I want and get an American commitment."

Until now Bush's Mideast team has been able to offer a fairly plausible explanation for the president's embrace of a plan that essentially cancels his own. As long as Arafat was around, they reasoned, the road map could go nowhere, since its initial requirements include Palestinian actions on security that Arafat refused to allow. Sharon's plan at least offered the prospect of a major positive movement, in Israeli withdrawal from Gaza. In the absence of an alternative, Bush could bet that Gaza would trigger political developments in both Israel and the Palestinian Authority that would make his two-state solution, rather than Sharon's long-term freeze, more likely.

With Arafat gone, that logic is undermined. The Palestinian warlord will be replaced, at least in the short term, by pro-peace moderates who have explicitly embraced the road map and its requirements. Should Mahmoud Abbas and Ahmed Qureia seek an immediate renewal of talks on implementation of the road map, and adopt the security steps Arafat had blocked, Sharon will lack a ready reason to refuse. More significant, Bush will come under considerable pressure to move forward on his own plan, which calls for the creation of a Palestinian state by the end of next year. Britain's Tony Blair responded to Bush's reelection last week by declaring that "the need to revitalize the Middle East peace process is the single most pressing political challenge in our world today."

Of course, Sharon may bet that the new Palestinian leadership will fail to act on the strategic opportunity before it or will be undermined by extremists. Or that Bush will shrink when the moment comes to challenge an Israeli leader. Given Palestinian history, and that of this White House, he may yet be safe.