Israeli Prime Minister Ariel Sharon promised Thursday a "broad and nonstop" retaliation a day after an Islamic Jihad suicide bomber set off an explosion killing at least five people and wounding dozens in the northern Israeli city of Hadera.
The attack highlighted how the small, hardened militia of Islamic fundamentalists has stepped to the forefront as the leading spoiler of a nine-month-old calm in violence between Israel and the Palestinians.
Of the four suicide attacks to strike inside Israeli cities this year, Islamic Jihad has claimed responsibility for the deadliest pair - Wednesday's explosion in an open-air market and the February bombing of a Tel Aviv nightclub that killed five.
The Damascus-headquartered radicals have stepped out of the shadow of the Islamic militant group Hamas, which has eased up a five-year bombing campaign for fear of sparking an escalation with Israel that would scuttle their campaign for Palestinian parliament.
That vote has sown at least a temporary division among gunmen who seek to gain influence at the ballot box and those who have chosen to remain outside of politics. The Associated Press reported that leading Islamic Jihad members have said privately their group continues to carry out attacks because it wants to be seen as less willing to compromise than Hamas.
"The calm has isolated not only Islamic Jihad, but all the philosophies of radicalism and violence," says Mohammed Dejani, a political science professor at Jerusalem's Al-Quds University. "They are hoping that Israel will respond so we'll return to square one."
Blaming the Palestinian Authority (PA) for not helping prevent the bombing, Mr. Sharon said Israel would act on its own to prevent terrorist attacks. An Israeli military spokesperson said the retaliation could include the first incursion into the Gaza Strip after Israel withdrew last month.
"The Palestinian Authority hasn't taken any serious step in the struggle against terrorism," Sharon said. "We aren't willing in any way for a continuation of terrorist acts, so our actions will be broad and nonstop until we bring about a cessation of terrorism."
A marginal player in Palestinian political and social life, Islamic Jihad's inspiration and orders mainly come from beyond the West Bank and Gaza, Israeli experts and officials say. The group's commanders are based in Syria while its ideology is inspired by Iran's fundamentalist regime.
While Hamas built up a social welfare organization alongside its armed wing to offer itself as political alternative to the ruling Fatah party, Islamic Jihad has remained focused on its military operations and isn't expected to run in the legislative election scheduled for January.
"Hamas has a much wider world view. Their goal is to have Islam ruling all echelons of life,'' says Mordechai Kedar, a former military intelligence officer and a research associate at Bar-Ilan University's Begin Sadat Center for Strategic Studies. "The Islamic Jihad doesn't bother with all those issues of social work, welfare, clinics, and food. They only care about the jihad, and everything else will be dealt with afterward."
Founded in Egypt in the 1970s, Palestinian Islamic Jihad was started as a radical splinter from the Muslim Brotherhood, the forerunner of Hamas. During the first Palestinian uprising a decade later, their leadership was exiled to Lebanon.
The group has a reputation for being small and highly secretive, making it less prone to infiltration by Israeli intelligence. It is believed to have been responsible for more than 40 attacks that have killed more than 100 Israelis, according to the Council on Foreign Relations.
Earlier this year the group joined a PA-brokered pact among militants to observe a tahadiyeh, Arabic for a state of calm, bowing to public sentiment that favored President Mahmoud Abbas's effort to restart peace talks with Israel.
But the group has made exceptions in order to retaliate against Israel's military offensives. The Israeli army's killing earlier this week of Islamic Jihad chief Louay Saadi in Tulkarem offered the militia a convenient trigger to fire rockets from Gaza into Israel and to launch the Hadera bombing, which wounded dozens.
The online edition of the Israeli newspaper Yediot Ahronot, Ynet, quoted an anonymous Islamic Jihad spokesperson from the northern West Bank who called the bombing a "natural" response to the killing of Mr. Saadi.
The Wednesday bombing was an affront to Mr. Abbas, who told the Palestinian legislature just hours earlier that armed groups who invite Israeli retaliation by ordering attacks were injuring the Palestinian people and the prospects for statehood.
Israeli spokespeople are using the bombing to spotlight the link between Palestinian militant groups and Iran. In a surprisingly serendipitous remark, President Mahmoud Ahmadinejad Wednesday praised Palestinian militants and expressed hope that their activities would help "wipe Israel from the map."
Shmuel Bar, a Middle East expert at the Herzliya Interdisciplinary Institute, says Islamic Jihad is used by Syria and the Iranians as a proxy. "From the point of view of the Iranians and the Syrians, to drag Israel into a military conflict in Gaza would take the heat off the Syrian issue," he said. Islamic Jihad "doesn't have very much to lose domestically."