AMONTH after promising President Bush that Israel would withdraw all of its troops and settlers from the Gaza Strip, Prime Minister Ariel Sharon is overseeing the largest military operation there in years -- yet another of the unpleasant surprises he has delivered to the administration as it struggles with a major crisis in Iraq. Since obtaining big diplomatic concessions from Mr. Bush in exchange for his Gaza plan -- at the cost of a significant backlash among America's dwindling number of Middle Eastern allies -- Mr. Sharon has further inflamed Arab opinion by assassinating the two top leaders of the Hamas movement while allowing his withdrawal plan to be halted by hard-liners in his Likud Party. Now he has dispatched a large Israeli military force to the southern end of the Gaza Strip for an offensive that has so far destroyed scores of Palestinian homes, with hundreds of more to be razed. If he proceeds, the cost of Mr. Bush's commitment to Israel's reckless leader will escalate yet again -- so far, with no return.

The motives for the Israeli offensive begin with response to the deaths of 13 soldiers in ambushes last week, many of them near the border between Gaza and Egypt. The bodies of some of the soldiers were dismembered by Palestinian attackers, and the Israelis are determined to retrieve the remains. There is also a tactical goal: By demolishing houses Israeli commanders hope to create a buffer zone along the border that will allow them to patrol it more safely and block tunnels through which arms and other contraband are smuggled into Gaza. Arguably such a measure would facilitate an eventual Israeli withdrawal from the rest of the territory. But the demolitions so far -- according to Israeli human rights monitors, 116 dwellings were destroyed over the weekend, leaving more than 1,000 people homeless -- have been carried out without regard for the welfare or possessions of desperately poor Palestinian residents. If the operation continues, thousands more will be made homeless.

The latest Israeli operation has gotten underway even as senior officials of the Bush administration have conducted a flurry of meetings with Palestinian and Arab leaders in an effort to demonstrate that the president's plan for an Israeli-Palestinian settlement has not been abandoned. Though they tried to repair some of the fallout from the president's commitment to Mr. Sharon, offering assurances that the United States has not prejudged the outcome of Israeli-Palestinian negotiations, Secretary of State Colin L. Powell and national security adviser Condoleezza Rice seemed to have no new ideas for how those talks might be started. Their conspicuously disparate reactions to Israel's actions also haven't changed: While Mr. Powell directly opposed them, Ms. Rice said only that they were "a concern."

In fact, as Mr. Powell conceded, the administration now finds itself in the uncomfortable position of waiting to see whether Mr. Sharon will take action to revive his Gaza withdrawal plan -- with part of the United States' own standing in the Middle East, and Iraq, hanging on the outcome. Normally a U.S. president might be expected to insist that his ally deliver on his promises instead of launching offensives that only make matters worse. But that has not been Mr. Bush's habit with Mr. Sharon.