FOR 18 MONTHS Israeli Prime Minister Ariel Sharon responded to Palestinian terrorist attacks by systematically destroying the infrastructure and institutions of the Palestinian Authority, all the while insisting that his intention was to pressure the very forces he targeted into cracking down on the terrorist groups. Three months ago his government moved beyond that strategy: It invaded the West Bank, crushed the remaining Palestinian forces there and assumed control over security itself. With the support of the Bush administration, Palestinian leader Yasser Arafat was declared irrelevant. Yet now, after two more suicide bombings by Muslim terrorists, Mr. Sharon has again responded by besieging Mr. Arafat. Because it is no longer possible for Mr. Sharon to argue that his purpose is to control terrorism, his aides are candid about an underlying aim: to drive Mr. Arafat and his closest collaborators back into exile. Instead, the prime minister has managed to restore Mr. Arafat's authority and support among Palestinians just as it was slipping away.

Mr. Sharon also has introduced complications into the Iraq campaign of his greatest ally, President Bush. The gratuitous siege in Ramallah grabbed the attention of the U.N. Security Council as Mr. Bush was seeking its support for a new resolution on Iraq; and Israel's refusal yesterday to respond to the council's order to pull back its forces, if sustained, will complicate U.S. attempts to win Arab and European backing for the enforcement of U.N. resolutions against Saddam Hussein. Administration spokesmen -- yesterday joined by the president himself -- have openly expressed chagrin at Mr. Sharon's "unhelpful" actions.

Yet Mr. Sharon and the hard-line leadership of the Israeli army may have good reason to risk White House irritation. In recent weeks -- despite the Bush administration's conspicuous lack of engagement -- there had been notable progress toward the formation of a reform-oriented Palestinian leadership that could supplant Mr. Arafat and renew a peace process. The Palestinian legislature this month forced Mr. Arafat's cabinet to resign; last week the reformers were close to forcing Mr. Arafat to accept the appointment of a respected moderate to the new post of prime minister. Meanwhile, Secretary of State Colin L. Powell put the Bush administration's imprimatur on a peace proposal drawn up by the multilateral Quartet group: It lays out a three-phase plan for creating a Palestinian state and includes a number of near-term measures Mr. Sharon strongly opposes, including the staging of Palestinian elections early next year, the withdrawal of Israeli forces from territories they have occupied during the past two years, and a halt to Jewish settlement construction.

It's hard to tell whether Mr. Bush genuinely supports the peace plan his State Department signed on to; his principal goal seems to be to avoid engagement in the Arab-Israeli conflict, or any hint of trouble with the Israeli government, whenever possible. Yet at least on paper, the Quartet process has aligned the United States, the United Nations, Russia, the European Union and most Arab governments behind a process opposed only by Mr. Sharon and Mr. Arafat. Mr. Sharon's latest attack on his nemesis has succeeded in freezing the process; if he continues, he may effectively block it altogether. That he is stepping on Mr. Bush's toes is unlikely to deter the Israeli warrior: His experience has taught him that this is a president he can push around.