Lebanon's Hezbollah has hurled itself into the war in Syria at the behest of its mentor Iran at the risk of damaging its reputation in the Arab world, experts say.
The powerful Shiite movement has won widespread support outside Lebanon for standing up to Israel on the battlefield.
But its involvement in Syria fighting alongside President Bashar al-Assad's troops against rebels has tarnished its reputation in the region, analysts told AFP.
"Hezbollah's participation in the war in Syria stems from an Iranian decision to support the regime until the end through regional actors, starting with Hezbollah," said Ziad Majed, professor in political sciences and the Middle East at the American University of Paris.
Hezbollah sent almost 1,700 fighters to the central Syrian town of Qusayr a week ago to support the regime's assault on the rebel stronghold.
Its chief Hassan Nasrallah had previously justified the organisation's involvement in Syria by saying they were defending Shiite villages and holy sites.
But the offensive on the mostly-Sunni town of Qusayr forced the movement to change its argument.
"Syria is the rear guard of the resistance (Hezbollah's fight with Israel), its backbone, and the resistance cannot stay with its arms folded when its rear guard is exposed," Nasrallah said on Saturday in a speech for the 13th anniversary of Israel's withdrawal from Lebanon.
"We are idiots if we do not act," he added.
Iran has asked Hezbollah to commit itself publicly to the conflict in Syria because its outcome could decide the future of the alliance between the two countries, and even of the region, said Ghassan al-Azzi, professor of political sciences at the Lebanese University.
"Hezbollah did it, even though it has affected its reputation in Lebanon and the Arab world," he added.
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But there are other reasons behind its military involvement, until now aimed exclusively at Israel.
"The fall of the Assad clan would have deadly consequences for Hezbollah, not only in terms of the free flow of weapons, men and money, but also because it would no longer enjoy the total support of the Syrian regime as it has over the past three decades," Waddah Sharara, sociology professor at the Lebanese University and author of a book on Hezbollah, told AFP.
But Majed said it is also a matter of pride for Hezbollah, which wants to be seen as "a key actor in the region and not just in Lebanon."
"It means that it is capable of intervening decisively in operations outside the country," he said.
In his speech on Saturday to mark Israel's withdrawal for which Hezbollah takes credit, Nasrallah said: "I have always promised you a victory and now I pledge to you a new one."
But it is not clear what the price of this victory would be.
Experts agree that Hezbollah's deployment of fighters to Syria has tarnished its image and, more seriously, aggravated tensions between Sunni and Shiite Muslims inside Lebanon itself.
"Hezbollah's reputation has suffered not only in the Arab world but also in Lebanon. Gone are the days when polls named Nasrallah as the most popular political leader in the Arab world for his resistance against Israel," after a 2006 war between Hezbollah and the Jewish state, Azzi said.
Majed said that Hezbollah was very conscious of its image in the Arab world until 2011, but since it realised the Syrian regime could fall, it has adopted a new outlook.
"It is only concerned about its image among its social base and in the short term, it has not weakened," he said.
"But we still do not know the danger Hezbollah is causing Lebanon by stirring tensions between Sunnis and Shiites," he added.
The reaction of the Sunni leader and former prime minister Saad Hariri to Nasrallah's speech was damning.
"You have excelled in sectarian incitement as never before. The time of exploiting Palestine, the resistance and national unity is gone. The Lebanese people, as well as the Arab and Islamic peoples, all know this," he said.