When Sheikh Hassan Nasrallah confirmed in May 2013 that Hezbollah had joined the Syrian Civil War on the side of President Bashar Al Assad's embattled government, it marked a major political and military shift for the Lebanese Shi'ite militia.

For the first time in decades, Hezbollah is now focused on fighting other Muslims rather than its sworn enemy, Israel. This new endeavor is also unique in that Hezbollah is fighting primarily in another country. Hezbollah has staged terrorist attacks in other countries before, but in Syria it's operating more or less as a traditional army rather than a guerrilla and terror group.

While committing to a foreign conflict is an unprecedented move for Hezbollah, it was not completely unpredictable. Hezbollah draws much of its financial and military support from Syria and Iran, so it makes sense that it would want to do everything it can to ensure the Assad regime remains in power.

Also, the prospect of sharing a border with Islamic State — should the Sunni militant group defeat the Syrian government's forces and other factions fighting for control of the country — would be troubling for Hezbollah and other Shi'ites in Lebanon. Islamic State's particular brand of extremist Sunni Islam casts Shias as apostates.

Hezbollah's involvement in Syria has caused considerable anxiety across Lebanon's southern border in Israel. The Israeli Defense Forces fear Hezbollah may come out of this conflict smarter, tougher, and better-equipped for any future confrontation with Israel.

In the years prior to their involvement in Syria, Hezbollah focused on domestic affairs, including power struggles with other groups within Lebanon, as well as on its ongoing conflict with Israel. From its inception in the early 1980s, Hezbollah's primary goals have been to transform Lebanon into an Islamic state and to fight against Israeli and Western influences in the region.

The group rose to prominence in Lebanon through the success of its military and civil ventures. Hezbollah provides its constituents schools, clinics and social security programs. As a consequence, the group remains influential throughout Lebanon despite being geographically isolated, to a considerable extent, in the country's Shia-dominant southern and northeast regions, where it has established a "state within a state."

Hezbollah's military wing has often been dismissed as merely a militia or terrorist organization. But despite its limitations it has proved to be a disciplined and effective fighting force. It was Hezbollah that finally forced Israel out of southern Lebanon in 2000 after 22 years of occupation. And when Israel invaded Lebanon again in 2006 following a cross-border attack by Hezbollah, the IDF faced far greater resistance from the militia than anyone had expected.

An Israeli soldier returning from battle against Hezbollah in 2006 told The New York Times that the Shi'ite fighters were highly trained. "All of us were kind of surprised," the soldier said. The IDF was also shocked to discover Hezbollah fighters equipped with advanced weaponry, flak jackets, and night-vision goggles. What shocked the Israeli military even more was how skilled Hezbollah fighters were with their anti-tank missiles.

"When Israel finally agreed to a ceasefire and began its withdrawal from the border area, it left behind upwards of 40 armored vehicles, nearly all of them destroyed by expertly deployed AT-3 ‘Sagger' anti-tank missiles," Alastair Crooke and Mark Perry wrote at CounterPunch. Israel lost 119 troops in three weeks of fighting.

Hezbollah suffered substantially greater casualties in the 2006 confrontation, with estimates ranging from 250 to more than 500 militia members killed. Israeli bombs and missile strikes also killed as many as 1,000 Lebanese civilians, while Hezbollah's rocket attacks on Israel killed 43 civilians. However, the fact that Hezbollah stood up to Israel's overwhelming might with only a few thousand of its own fighters earned the militia greater admiration in Lebanon. Its ranks and coffers swelled.

By the time Hezbollah officially entered the Syrian Civil War in 2013, it was more powerful than it had ever been. "The movement's military structure is based on an elite force backed by a full-time militia and a large corps of part-time reserves who undergo rudimentary weapons training — often in Iran — but have jobs outside the group," an analyst told Reuters. "[A]ltogether the total force including the part-time men, known as Saraya, reached 50,000, of which 10,000 to 15,000 were elite forces.

Despite its newfound strength and the desire to support its patron Syria and prevent Islamic State from gaining control of its neighbor, the decision to enter the conflict did not come lightly for Hezbollah's command. As early as 2012, there were rumors that Hezbollah soldiers were covertly training and supporting Al Assad's regime. However, Hezbollah denied playing any direct combat role in Syria until its 2013 announcement.

But as the fighting in Syria escalated, so did Hezbollah's mission there. Today there are as many as 8,000 Hezbollah fighters on the ground in Syria, potentially twice as many as fought Israel in 2006. This could suggest a significant increase in Hezbollah's military capabilities.

One of the main worries for Israel is that Hezbollah has been training and fighting alongside Syrian government forces as well as Russian and Iranian forces, even taking the lead in some engagements. At the Battle of Qusair, Hezbollah fighters recaptured a strategically important border town from the Free Syrian Army following a three-week siege. One observer referred to the clash as "defining battle of the country's civil war."

Of particular concern to Israel is the expertise Hezbollah is gaining from fighting alongside Russian advisers in Syria. "For the first time in its history, Hezbollah is conducting offensive-maneuver warfare as part of its operations in Syria," Nadav Pollak and IDF Brig. Gen. Muni Katz wrote in a December report from the Washington Institute. "The Russian intervention is only enhancing that experience, likely giving the group important lessons for future conflicts."

"In Syria, Hezbollah has had to shift its main objectives to taking over territory and maintaining control over it, all while fighting quasi-conventional forces that use guerrilla tactics," the report continues. "Against the IDF, the group was accustomed to fighting in small units on familiar terrain, but now it is deploying hundreds of fighters in complex offensive operations on unfamiliar territory. For Hezbollah's commanders and fighters, such experience can change their views on the most effective way to win a battle, and Russia's involvement means that they are learning such lessons from one of the best militaries in the world."

The report concludes that the experience Hezbollah is gaining from Russia and Iran has made it one of the most effective fighting forces in the tangled Syrian conflict. Hezbollah fighters are getting so good that they're reportedly growing frustrated by what they consider to be an inept Syrian army.

Israel is somewhat hamstrung when it comes to taking action against Hezbollah forces embedded with Russian and Iranian troops, as doing so could potentially escalate an already delicate international balance. But the Israeli air force has apparently interdicted suspected arms shipments headed for Hezbollah's contingents in Syria.

"Israel has struck weapons consignments in Syria on eight occasions since January 2013, hitting an array of sophisticated weaponry, including Iranian Fateh-110 guided missiles, SA-8 and SA-17 anti-aircraft systems, and Yakhont anti-ship cruise missiles," Al Jazeera reported. "In each case, the weapons were allegedly earmarked for transfer to the Lebanese organization Hezbollah, a Shia group backed by Iran."

One of the bombing raids targeted an airstrip in Lebanon's northern Bekaa Valley that Israel reportedly suspected of housing Hezbollah's drones. The group's robots also underscore "the technical advances of the organization, with its growing arsenal of guided missiles and the challenges it poses to Israel in a future war," Al Jazeera noted.

Despite Israel's efforts to prevent Syrian weapons from falling into Hezbollah's hands, there are reports that the Assad regime has smuggled some long-range guided missiles and other weaponry to the militia. Most notably, the United States and others believe that Hezbollah is now in possession of up to 12 of the Yakhont anti-ship cruise missiles, in addition to its massive stockpile of other munitions. "It is generally accepted that Hezbollah now has an arsenal of roughly 100,000 thousand rockets and missiles, many of them capable of striking deep within Israel," Foreign Policy reported.

That's vastly more, and better, rockets than Hezbollah possessed during its 2006 war. "We need to prepare for the possibility of a ‘blitz' which could lead to between 1,000 to 1,500 rockets falling on Israel daily," IDF Maj. Gen. Eyal Eizenberg told Haaretz.

It's understandable that Israel now considers Hezbollah to be one of the Jewish state's greatest military threats. If Hezbollah comes out of the Syrian Civil War intact, it will be a stronger, battle-hardened fighting force with a massive stockpile of missiles. Israel will remain militarily dominant in many regards, but any future conflict between the two powers could prove far more expensive — in both money and blood — for the Jewish state compared to previous clashes.

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