JERUSALEM, June 10 (JTA) — A recent terrorist attack in the Gaza Strip that killed four Israeli soldiers represented a bloody demonstration of unity among the three leading Palestinian terrorist groups — Hamas, Islamic Jihad and the Al-Aksa Martyrs Brigade. Despite past differences, the various terrorist organizations have increased their cooperation in recent years, particularly in the face of Israel’s counter-terror operations. HAMAS: The largest opposition party in the West Bank and Gaza, Hamas’ ideology focusing on the destruction of Israel is based on jihad, the Muslim “holy war” against the heathens. Hamas terrorists in the Izz a-Din al-Kassam Brigade, the organization’s military wing, have conducted many attacks — including large-scale suicide bombings — against Israeli civilian and military targets. In the early 1990s, Hamas also targeted suspected Palestinian collaborators and rivals in the Fatah movement. The group has not specifically targeted U.S. interests, though some American citizens have been killed in Hamas operations. Like Islamic Jihad, Hamas has its origins in the Muslim Brotherhood, which was founded in Egypt in the first half of the 20th century. The Muslim Brotherhood is considered the ideological forerunner of many fundamentalist Muslim organizations. The spiritual leader of Hamas is Sheik Ahmed Yassin, 66, who was paralyzed following an accident in his youth. Yassin founded the Islamic Center in Gaza in 1973, turning it not only into a major religious organization but also the basis for a network of social institutions — including welfare, education and medical institutions — that increased the movement’s popularity. That paved the way for the founding of Hamas after the first Palestinian intifada began in 1987. In 1989, Yassin was arrested by Israel and sentenced to life imprisonment for ordering the killing of Palestinians who allegedly had collaborated with the Israeli army. He was released in 1997 in exchange for two Israeli Mossad agents captured during an assassination attempt on a Hamas leader in Jordan. The group’s leadership is dispersed throughout the Gaza Strip and West Bank, with a few senior leaders in Syria, Lebanon and the Persian Gulf states. Hamas receives some funding from Iran, but relies primarily on donations from Palestinian expatriates around the world and private benefactors in moderate Arab states. Some of Hamas’ fund-raising and propaganda activity takes place in Western Europe and North America. Israeli intelligence in the past has pointed at possible links between Hamas and Al-Qaida and Hezbollah, but so far those ties have not been proved. ISLAMIC JIHAD: This fundamentalist group was inspired by the Iranian Revolution of 1979. Islamic Jihad is a coalition of several radical Islamic factions that became active after 1979 in the West Bank, mainly under the influence of the Iranian Islamic revolution and the growing Islamic militancy in the region. It, too, aspires to destroy Israel as part of a jihadist “holy war” to impose the rule of Islam in the world. The group carried out its first terror attacks in mid-1986, before the first intifada began. The organization is led by Ramadan Shallah, who is based in Damascus. It receives financial assistance from Iran and limited logistic assistance from Syria. The group operates primarily in Israel, the West Bank and Gaza Strip, but many of the group’s leaders reside in other parts of the Middle East, including Lebanon and Syria. In August 1988, the group’s leaders were expelled to Lebanon, where Fathi Shqaqi reorganized the group and strengthened its ties with Hezbollah and Iran. Unknown assailants killed Shqaqi in October 1995 in Malta. But Islamic Jihad regrouped for several of the deadliest terrorist attacks carried out in Israel by radical Islamic organizations from 1995 to 1997, which helped destroy the Oslo peace process. The AL-AKSA MARTYRS BRIGADE: The organization stems from the Palestinian Authority’s ruling Fatah movement, to which both P.A. President Yasser Arafat and Prime Minister Mahmoud Abbas belong, and its radical, younger offspring, the Tanzim, which was run by Marwan Barghouti until he was arrested by Israeli troops last year. The brigade emerged as a terrorist group following the outbreak of the current intifada in September 2000, only after Arafat apparently gave the green light for armed struggle against Israel. After that it received manpower and funds from Fatah. Since that time, the brigade has carried out numerous terrorist attacks in Israel and the West Bank. Documents seized in a spring 2002 raid on Arafat’s headquarters included an invoice from the brigade asking for reimbursement for explosives used in bombings in Israeli cities, Israeli military sources said. The document was addressed to Brig. Gen. Fuad Shubaki, the Palestinian Authority’s chief financial officer for military operations, and contained numerous handwritten notes and calculations. The document was the first direct proof of what Israel’s intelligence establishment has claimed for some time: that the Al-Aksa Martyrs Brigade is not a “rogue militia” as Arafat claims. Rather, its members are on the Palestinian Authority’s payroll, its activities are financed from Palestinian Authority coffers and its attacks are carried out with the knowledge and backing of Arafat’s inner circle, Israel says. The U.S. government hesitated to probe deeply into the activities of the organization, in order not to alienate the Palestinian Authority as a possible peace partner, but the State Department has added the group to its list of foreign terrorist organizations.