By Mariam Karouny

BEIRUT (Reuters) - Conflict in Syria kills hundreds of thousands of people and spreads unrest across the Middle East. Iranian forces battle anti-Shi'ite fighters in Damascus, and the region braces for an ultimate showdown.

If the scenario sounds familiar to an anxious world watching Syria's devastating civil war, it resonates even more with Sunni and Shi'ite fighters on the frontlines - who believe it was all foretold in 7th Century prophecies.

From the first outbreak of the crisis in the southern city of Deraa to apocalyptic forecasts of a Middle East soaked in blood, many combatants on both sides of the conflict say its path was set 1,400 years ago in the sayings of the Prophet Mohammad and his followers.

Among those many thousands of sayings, or hadith, are accounts which refer to the confrontation of two huge Islamic armies in Syria, a great battle near Damascus, and intervention from the north and west of the country.

The power of those prophecies for many fighters on the ground means that the three-year-old conflict is more deeply rooted - and far tougher to resolve - than a simple power struggle between President Bashar al-Assad and his rebel foes.

Syria's war has killed more than 140,000 people, driven millions from their homes and left many more dependent on aid. Diplomatic efforts, focused on the political rather than religious factors driving the conflict, have made no headway.

"If you think all these mujahideen came from across the world to fight Assad, you're mistaken," said a Sunni Muslim jihadi who uses the name Abu Omar and fights in one of the many anti-Assad Islamist brigades in Aleppo.

"They are all here as promised by the Prophet. This is the war he promised - it is the Grand Battle," he told Reuters, using a word which can also be translated as slaughter.

On the other side, many Shi'ites from Lebanon, Iraq and Iran are drawn to the war because they believe it paves the way for the return of Imam Mahdi - a descendent of the Prophet who vanished 1,000 years ago and who will re-emerge at a time of war to establish global Islamic rule before the end of the world.

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According to Shi'ite tradition, an early sign of his return came with the 1979 Iranian revolution, which set up an Islamic state to provide fighters for an army led by the Mahdi to wage war in Syria after sweeping through the Middle East.

"This Islamic Revolution, based on the narratives that we have received from the prophet and imams, is the prelude to the appearance of the Mahdi," Iranian cleric and parliamentarian Ruhollah Hosseinian said last year.

He cited comments by an eighth century Shi'ite imam who said another sign of the Mahdi's return would be a battle involving warriors fighting under a yellow banner - the color associated with Lebanon's pro-Assad Hezbollah militia.

"As Imam Sadeq has stated, when the (forces) with yellow flags fight anti-Shi'ites in Damascus and Iranian forces join them, this is a prelude and a sign of the coming of his holiness," Hosseinian was quoted as saying by Fars news agency.

MEDIEVAL BATTLEFIELDS

Islam split into its Sunni and Shi'ite branches during a war over the succession to the leadership of the faith in the generation that followed the Prophet Mohammad's death in 632.

The hadith, or sayings of the prophet and his companions, have been handed down orally over the centuries and are the most important sources of authority in Islam after the Quran itself. Many date back to those medieval battlefields in what are now Syria and Iraq, where the two main Islamic sects took shape.

The historical texts have become a powerful recruitment tool, quoted across the region from religious festivals in Iraq's Shi'ite shrine city of Kerbala to videos released by Sunni preachers in the Gulf, and beyond.

"We have here mujahideen from Russia, America, the Philippines, China, Germany, Belgium, Sudan, India and Yemen and other places," said Sami, a Sunni rebel fighter in northern Syria. "They are here because this what the Prophet said and promised, the Grand Battle is happening."

Both sides emphasize the ultimate goal of establishing an Islamic state which will rule the world before total chaos.

Although some Sunni and Shi'ite clerics are privately skeptical of the religious justifications for the war, few in the region express such reservations in public for fear of being misinterpreted as doubters of the prophecies.

"Yes some of the signs are similar but these signs could apply at any time after the fall of the Islamic state (1,000 years ago)," one Sunni Muslim scholar in Lebanon said, asking that he not be identified. "There is no way to confirm we are living those times. We have to wait and see."

For the faithful, the hadith chart the course of Syria's conflict from its beginning in March 2011, when protests erupted over the alleged torture of students and schoolboys who wrote anti-Assad graffiti on a school wall in Deraa.

"There will be a strife in Sham (Syria) that begins with children playing, after which nothing can be fixed," according to one hadith. "When it calms down from one side, it ignites from the other."

Hadith on both sides mention Syria as a main battlefield, naming cities and towns where blood will be spilled.

Hundreds of thousands of people will be killed. The whole region will be shaken from the Arabian Peninsula to Iraq, Iran and Jerusalem, according to some texts.

Saudi Arabia will collapse. Almost every country in the Middle East will face unrest. One statement says "blood will reach knee-level".

A widely circulated hadith attributed to Mohammad says Sham, or Syria, is God's favored land. Asked where the next jihad will be, he replies: "Go for Sham, and if you can't, go for Yemen ...(though) God has guaranteed me Sham and its people."

Another refers to Muslims gathering "at the time of war in Ghouta, near a city called Damascus". Ghouta, east of Syria's capital, has been a rebel stronghold for the last two years.

A Sunni hadith speaks of a battle in a town called Dabeq, in northern Syria near the Turkish border, and intervention by a foreign army to split the Muslim fighters - seen by some as a reference to a possible Turkish incursion.

ARAB SPRING GIVES WAY TO SECTARIAN STRUGGLE

Syria's civil war grew out of the "Arab Spring" of pro-democracy revolts in the Middle East and North Africa in 2011 after Assad's forces cracked down hard on peaceful protests.

But because Assad is a member of the Alawite sect, an offshoot of Shii'ism, and most of his opponents are Sunni Muslims, the fighting quickly took on a sectarian character, which has largely overwhelmed the political issues.

"These hadith are what the Mujahideen are guided by to come to Syria, we are fighting for this. With every passing day we know that we are living the days that the Prophet talked about," said Mussab, a fighter from the Nusra Front, a Sunni hardline group linked to al Qaeda, speaking from Syria.

Murtada, a 27-year-old Lebanese Shi'ite who regularly goes to Syria to battle against the rebels, says he is not fighting for Assad, but for the Mahdi, also known as the Imam.

"Even if I am martyred now, when he appears I will be reborn to fight among his army, I will be his soldier," he told Reuters in Lebanon.

Murtada, who has fought in Damascus and in the decisive battle last year for the border town of Qusair, leaves his wife and two children when he goes to fight in Syria: "Nothing is more precious than the Imam, even my family. It is our duty."

Syria's civil war built upon sectarian conflicts elsewhere, especially in Iraq and Lebanon, leading to a growing sense across the region that all those power struggles in individual countries were part of a titanic battle for the future.

Abbas, a 24-year-old Iraqi Shi'ite fighter, said he knew he was living in the era of the Mahdi's return when the United States and Britain invaded Iraq in 2003.

"That was the first sign and then everything else followed," he told Reuters from Baghdad, where he said was resting before heading to Syria for a fourth time.

"I was waiting for the day when I will fight in Syria. Thank God he chose me to be one of the Imam's soldiers."

Abu Hsaasan, a 65 year old pensioner from south Lebanon, said he once thought the prophecies of the end of days would take centuries to come about.

"Things are moving fast. I never thought that I would be living the days of the Imam. Now, with every passing day I am more and more convinced that it is only a matter of few years before he appears."

(Editing by Dominic Evans and Peter Graff)