By Laila Bassam and Tom Perry
BEIRUT (Reuters) - When Iran declared victory over Islamic State in Syria and Iraq, it hailed the "strong and pivotal" role played by Lebanon's Hezbollah movement.
The praise, contained in a top general's letter to Iran's Supreme Leader in November, confirmed Hezbollan's pre-eminence among Shi'ite Muslim regional groups backed by Tehran that are helping the Islamic Republic exert influence in the Middle East.
Hezbollah has emerged as a big winner in the turmoil that has swept the Arab world since the uprisings of 2011 that toppled governments in several countries. It has fought in Syria and Iraq, trained other groups in those countries and inspired other forces such as Iran-allied Houthis waging a war in Yemen.
But its growing strength has contributed to a sharp rise in regional tension, alarming Israel, the United States - which designates it as a terrorist organization - and Sunni Muslim monarchy Saudi Arabia, Iran's regional rival, which accuses Hezbollah of having a military role on its doorstep in Yemen.
Israel fears Iran and Hezbollah will keep permanent garrisons in Syria and has called for action against "Iranian aggression". With Hezbollah stronger than ever, war with Israel is seen by many in the region as inevitable, sooner or later.
"Hezbollah has gained from the experience of working with armies and managing numerous weapons systems simultaneously - air power, armored vehicles, intelligence, and drones: all specialties of conventional armies," said a commander in a regional alliance fighting in Syria.
"Hezbollah is now a dynamic army, bringing together guerrilla and conventional warfare."
Hezbollah's elevated status among Iran's regional allies was clear at the funeral this month of Hassan Soleimani, father of Major General Qassem Soleimani who wrote the letter praising Hezbollah's role fighting IS in Syria and Iraq.
Hezbollah's delegation, led by Sayyed Hashem Safieddine, a top figure in its clerical leadership, took responsibility for organizing talks on the sidelines of the funeral between the various Iranian allies present, an official who attended said.
"All the resistance factions were at the condolences. Hezbollah coordinated and directed meetings and discussions," the official said.
THE "HEZBOLLAH MODEL"
Hezbollah was set up by the powerful Iranian Revolutionary Guards (IRGC) to fight Israeli forces that invaded Lebanon in 1982 and to export Iran's Shi'ite Islamist revolution.
It has come a long way from the Bekaa Valley camps where its fighters first trained. Its fighters spearheaded the November attack on Albu Kamal, a town near Syria's border with Iraq, which ended IS resistance in its last urban stronghold in the country.
Hezbollah leader Sayyed Hassan Nasrallah has said the battle for Albu Kamal was led by Qassem Soleimani, commander of the branch of the IRGC responsible for operations outside.
An Iran-backed Iraqi Shi'ite militia, the Popular Mobilisation Forces (PMF), crossed into Syria to help during the battle. Hezbollah helped to set up the Iraqi PMF at the peak of Islamic State's expansion in 2014.
The attack was of huge symbolic and strategic significance for Iran and its regional allies, recreating a land route linking Tehran, Baghdad, Damascus and Beirut - often termed the "Shi'ite crescent" by Iran's regional enemies.
The United States says Iran is "applying what you might call a Hezbollah model to the Middle East - in which they want governments to be weak, they want governments to be dependent on Iran for support," White House National Security Adviser H.R. McMaster said in late October.
"So, what is most important, not just for the United States but for all nations, is to confront the scourge of Hezbollah and to confront the scourge of the Iranians and the IRGC who sustain Hezbollah's operations," he told Alhurra, a U.S.-funded Arabic-language news network.
Syria is where Hezbollah has made its biggest impact outside Lebanon though its role was kept secret when its fighters first deployed there Syria in mid-2012.
The initial aim was to defend the shrine of Sayeda Zeinab, a Shi'ite pilgrimage site near Damascus. But as President Bashar al-Assad lost ground, Hezbollah sent more fighters to aid Syrian security forces ill suited to the conflict they faced.
Hezbollah's role was crucial in defeating many of the rebels who fought Assad with backing from his regional foes, helping him win back the cities of Aleppo and Homs, and other territory.
Its publicly declared role in support of Assad has been accompanied by an effort to establish new Syrian militias that have fought alongside it, said the commander in the regional alliance fighting in Syria.
Hezbollah has lost more than 1,500 fighters in Syria, including top commanders. But it has gained military experience, supplementing its know-how in guerrilla tactics with knowledge of conventional warfare thanks to coordination with the Syrian and Russian armies and the IRGC, the commander said.
A "BROADENING THREAT"
With Iranian support, Hezbollah has raised and trained new Syrian militias including the National Defence Forces, which number in the tens of thousands, and a Shi'ite militia known as the Rida force, recruited from Shi'ite villages, the commander said.
Hezbollah has also taken the lead in the information war with a military news service that often reports on battles before Syrian state media.
The United States and Saudi Arabia are worried Hezbollah and Iran are seeking to replicate their strategy in Yemen, by supporting the Houthis against a Riyadh-led military coalition.
Hezbollah denies fighting in Yemen, sending weapons to the Houthis, or firing rockets at Saudi Arabia from Yemeni territory. But it does not hide its political support for the Houthi cause.
Saudi concern over Yemen is at the heart of a political crisis that rocked Lebanon in November. Lebanese Prime Minister Saad al-Hariri's sudden resignation was widely seen as a Saudi-orchestrated move to create trouble for Hezbollah at home.
Shared concerns over Hezbollah may have been a motivating factor behind recently declared contacts between Saudi Arabia and Israel.
Hezbollah is meanwhile expanding its conventional arsenal in Lebanon, where it is part of the government, including buying advanced rocket and missile technology, in "a broadening of the threat to the Eastern Mediterranean and the Arabian Peninsula", Nick Rasmussen, the director of the U.S. National Counterterrorism Center, said in October.
Despite newly imposed U.S. sanctions, Hezbollah sounds confident. With IS now defeated in Iraq, Nasrallah has indicated Hezbollah could withdraw its men from that front, saying they would "return to join any other theater where they are needed".
He says his group will continue to operate wherever it sees fit, repeatedly declaring: "We will be where we need to be."
(Additional reporting by Parisa Hafezi in Ankara; Writing by Tom Perry, Editing by Timothy Heritage)