BEIRUT--The name Ain al-Hilweh means "Sweet Spring" in Arabic, but to 70,000 Palestinians it describes a crowded, impoverished refugee camp ringed by Lebanese Army checkpoints and tanks. The four checkpoints, the only ways in and out of the square-mile slum, are deemed necessary because more than 20 armed factions compete for influence in what has always been the largest and toughest Palestinian camp in Lebanon.

It's a conflict zone now on the verge of spilling out into the neighboring Lebanese city of Sidon, as radical jihadists return from wars in Afghanistan, Chechnya, and Iraq imbued with an Islamist extremism that is drawing more recruits and changing the complexion of the once secular Palestinian movement. The camp, say Palestinian and Lebanese officials, has supplied scores of fighters to the Iraq insurgency, particularly the terrorist organization that was headed by Abu Musab Zarqawi.

Islamist powerbase. Sitting in his office in Sidon, a senior Lebanese military intelligence official pores over an aerial map of the camp covered in small stickers that show the general location of militant groups. But the Lebanese Army can't enter the area, where well-armed Palestinian militias of mainstream Fatah, rival Hamas, and several Islamist groups rule the streets and frequently clash in gunfights. And the Army has had to concede an adjacent neighborhood to armed groups of radical Islamists considered aligned with al Qaeda: Jund al-Sham (Army of Greater Syria), a mostly Lebanese group originated by veterans of the war against the Soviets in Afghanistan, and Asbat al-Ansar (League of Partisans), which is mostly Palestinian.

In any Palestinian camp or neighborhood, the walls are adorned with posters depicting "martyrs" of the fight against Israel. But in Asbat's neighborhood, the Iraq battlefield is evident: The main road has been renamed "Martyrs of Fallujah," and the signs glorify men killed fighting alongside Zarqawi or in suicide attacks against U.S. troops or Iraqi Shiite Muslims.

One Lebanese member of Jund al-Sham says that these groups are aligned with al Qaeda in the sense that they share a worldview of Salafism, or return to the most basic principles of Islam, and the need for jihad to free Muslim lands from infidel occupiers. The Iraq war, says Abed al-Jalil (who insisted his real name not be used), helped strengthen the jihadist group in Lebanon, which had been plagued by infighting and constrained by Lebanese and Syrian authorities. "Before, there were Salafists, Takfiris, Wahabbis who all disagreed on minor points and did not unify," he says. "But now, they are one."

By his account, Jalil spent part of the summer in 2004 living, training, and fighting with Zarqawi's fighters in Fallujah. He says he planned to conduct a suicide attack but was sent back to Lebanon because his education made him valuable as a recruiter. "I hope to have the heart to be a martyr," says Jalil, whose story can't be independently verified. "Right now, I am struggling with whether the dawa [preaching] is stronger than the bullet."

Sheik Maher Hammoud, a Salafist cleric in Sidon who preaches in a mosque just outside the camp, explains the need for good Muslims to fight what he regards as the American occupation of Arab lands. While not a member of Asbat, Hammoud has contacts in the group. "The question is not why they would go and fight in Iraq," he says. "It's why they would not go."