When Palestinian security forces moved into the lawless West Bank towns of Jenin and Nablus, all they had to worry about were the armed criminal gangs who had been shaking down shopkeepers and stealing cars — it didn't take long to wrest control from the thugs. But Hebron, where 600 Palestinian forces rolled up over the weekend in shiny new white pick-up trucks, is far more dangerous, because it is a stronghold of Hamas and also the base of an extremist Jewish settler community. The Islamists see the new paramilitary unit as a U.S.- and Israeli-built proxy force to be used against them; while the settlers see the Palestinian security men as "terrorists in uniform," and are threatening an armed showdown.

The theory behind the new Palestinian security force — whose months of training by Jordanian police was supported and funded by the U.S. State Department, with a U.S. security coordinator, General Keith Dayton, acting as adviser to the Palestinians — is to persuade skeptical Israelis that Palestinians can ensure security inside the West Bank, a first step in building trust that would allow Israel to turn over control to the Palestinian Authority as part of any peace agreement. So far, the plan has succeeded: The streets are safer for ordinary Palestinians, and Israelis have more confidence in their Palestinian security counterparts. But it's precisely because this unit forms part of a plan to prepare the Palestinians to take control of the area that the Hebron settlers are so vehemently opposed — they have no interest in a two-state solution that would transfer land that they see at their Biblical birthright to Palestinian control.

The ancient city of 170,000 Palestinians is a tinder box of religious passions. It is the site of the Patriarchs' Cave, revered by Muslims, Jews and Christians as the tomb of Abraham and his wife, Sarah. And nearly 700 Jewish settlers are squatting near the tomb, protected by hundreds more Israeli soldiers whose security barriers and checkpoints have paralyzed the heart of this Palestinian city. The effect of the settler presence, with its attendant security measures, has been to radicalize the city's Palestinian population.

Many Hebronites now support the armed resistance preached by Islamic militants, and although Hamas has been driven underground by Israeli raids and arrests, it remains a powerful force in the city. Hebron sources told TIME that Hamas will try to sabotage the deployment of the new Palestinian security forces by staging attacks on the Israeli settlers or military, which could lead to a backlash against the U.S.-sponsored security force. Says a Hamas spokesman: "We are opposed to the Palestinian security apparatus. Its aim is to serve Israeli interests by arresting Hamas activists." But with Israeli forces raiding suspected Hamas hideouts in the West Bank every night, the Islamists may not have the weapons and men to strike back.

The arriving Palestinian commanders got a foretaste of Hebron's explosive nature on Saturday night when Jewish settlers went on a rampage after Israeli police, backed by the Israel Defense Forces, destroyed houses erected by several militant Jewish settlers inside a Palestinian-owned olive grove. The settlers, many of them armed, swarmed into a Palestinian neighborhood, smashing cars and vandalizing a Muslim cemetery.

Furious settlers later released a statement condemning the Israeli security forces. "We hope they will be defeated by their enemies, that they will all be [kidnapped IDF soldier] Gilad Shalit, that they will all be killed and all slaughtered because this is what they deserve," it read. Settler wrath was also aimed at Washington. Commenting on the arrival of the U.S.-sponsored Palestinian security forces in Hebron, settler leader Baruch Marzel told TIME: "It's like asking Bin Laden's men to come protect Manhattan." He added: "They're terrorists. We'll shoot them if they come near our houses."

To keep that from happening, the Palestinian forces will be deployed in the southern and western sides of Hebron, away from direct contact with the settlers. But lately, Jewish extremists have been straying from their gated outposts and attacking Palestinian shepherds and families harvesting the olive groves. "We're not worried," says Gen. Diab Ali, commander of Palestinian security forces in the West Bank. "We are counting on the Israeli army to control the settlers." One senior Israeli officer concurs. "The Palestinians won't be close to the settlements. They'll be confronting Hamas, and this is in our best interests." The Palestinian forces, says Ali, will try to quell drug trafficking, car theft and bring an end to Hebron's murderous clan feuds.

As for Hamas, says the Palestinian commander, "We're here to bring law and order to Hebron, and that means confiscating weapons from everybody, even the political groups." The U.S.-supported Palestinian force may be armed and well-disciplined, but in Hebron that may not be enough. With Jewish settlers and Hamas gunning for them, the Palestinian officers will need to display caution and diplomacy, too.