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The Lebanese militant group Hezbollah is at the center of the recent crisis that has gripped Lebanon and rattled a region already rife with conflict.

When Saudi-backed Prime Minister Saad Hariri declared his resignation in a surprise announcement from the Saudi capital, Riyadh, he blamed Hezbollah for imposing itself on the country and doing the bidding of its main backer, Iran, in Lebanon and elsewhere in the region.

The one-time local Shiite guerrilla army that rallied Lebanon's Shiites and battled Israel — even earning admiration from the region's Sunnis— has turned into a powerful, well-armed group caught up in the Iran-Saudi rivalry that is shaping the Middle East.

Saudi Arabia singled Hezbollah out, accusing it of declaring war on the kingdom, just as the U.S. ratcheted up its pressure on Iran and imposed new sanctions on Hezbollah, which it considers a terrorist group.

Here is a look at the 35-year old militant group, its sources of power and regional role.

BEGINNINGS and LEADERS

Hezbollah was formed by the Iranian Revolutionary Guard in 1982 to fight Israel's invasion of Beirut. Under the leadership of Hassan Nasrallah, who took over in 1992 after his predecessor, Abbas Mussawi, was killed in an Israeli airstrike, the group moved from seeking to implement an Iranian-style Islamic republic in Lebanon to focusing on fighting Israel and integration into Lebanon's sectarian-based politics. Nasrallah, now 57, has played a key role in ending a feud among Shiites, focusing attention toward fighting Israel and later expanding the group's regional reach.

WARS WITH ISRAEL

Hezbollah became the main militant group opposing Israel and the buffer zone it established in southern Lebanon in 1985. In 1996, Israel launched an offensive to end the guerrilla attacks, striking Lebanese power stations and killing more than 100 Lebanese civilians sheltering in a U.N. base. A year later, 12 Israeli soldiers were killed in a commando raid in the south.

Israel's withdrawal from southern Lebanon in 2000 was hailed as a victory for Hezbollah. In the same year, Hezbollah captured three Israeli soldiers and a businessman in cross border raids, and later negotiated a swap in 2004, releasing hundreds of prisoners and fighters.

Then in 2006, Hezbollah captured two Israeli soldiers in a cross-border raid, sparking a 34-day war that killed 159 Israelis and more than 1,000 Lebanese.

A U.N.-brokered cease-fire brought thousands of international peacekeeping troops to police the Israeli-Lebanese border.

HEZBOLLAH AT HOME

Hezbollah's popularity at home didn't only stem from its opposition to Israel. With a weak Lebanese state, the Iranian-sponsored group, like most other sects, provided a vast array of social services for its supporters, through education, health and social networks. But as the militant group sought more executive and legislative powers following Israel's 2000 withdrawal, it worked to funnel some of its support through state institutions to reach the broader public.

Another turning point came in 2008, when heavily armed Hezbollah fighters seized control of vast parts of Beirut, flexing its power domestically for the first time. The show of force followed attempts by Lebanon's Western-backed government to curb the militants' influence by dismantling its telecommunication network.

Hezbollah has been the most powerful player in Lebanon's politics ever since. Saudi Arabia and Iran, which backed opposite sides inside Lebanon, ended a two-year deadlock over electing a president by tacitly approving a power-sharing deal that effectively enshrined Hezbollah's new powerful role. With that, Hariri, a Sunni, headed a unity government and Michel Aoun, a Hezbollah ally, became president.

REGIONAL FOOTPRINT

Hezbollah sent its gunmen to fight alongside the forces of Syrian President Bashar Assad in 2012, providing a significant boost to the overstretched military, and turning the tide of the war. It was also crucial to safeguarding the Shiite militant group's access to Syria, the land corridor through which it is believed to get its weapons from Iran.

Citing estimates based on diplomatic reports and open-source data, Thanassis Cambanis, a fellow at the Century Foundation, said some 20,000 to 30,000 armed men, including 4,000 core fighters as well as local militiamen and tribesmen are under Hezbollah's command in the fighting in Syria and Lebanon.

This war has allowed Hezbollah's fighters to improve their interoperability, working closely with the Russian military and other Iranian-backed militias from Iraq or Afghanistan.

Hezbollah is also believed to have increased its military facilities in Syria's Qalamoun mountains and in the Golan Heights, as well as throughout Lebanon. It is believed to have built munitions factories there and Israeli officials estimate it has an arsenal of 150,000 missiles.

"It has crossed a military threshold," Cambanis said. "Hezbollah today possesses a credible deterrent against pre-emptive war by its opponents."

Hezbollah backing for Yemen's Houthi rebels has been harder to prove, though the structure and rhetoric of the Shiite Yemeni group mirrors that of Hezbollah. Saudi Arabia accuses Hezbollah and Iran of providing the Houthis with training and financial support in the stalemated war that is causing a humanitarian disaster.

Saudi authorities said a recently intercepted missile near Riyadh airport, the longest-range yet used by the Houthis, had Iranian markings, confirmed by the Americans. In addition, small arms shipments allegedly from Iran were confiscated before reaching Yemen. And Saudi TV networks have aired what they say was evidence of Hezbollah fighters training the Houthis.