(The name of the reporter has been withheld for security reasons)

HOMS COUNTRYSIDE, Syria (Reuters) - For months, the western rural region of Homs was an unusual model of coexistence in Syria's brutal two-year-old civil war. Now, it risks becoming a dark episode in the country's deepening sectarian conflict.

Syria's uprising-turned-war has forced most Syrians to take sides in a struggle that has killed more than 100,000.

But in this strategic stretch of territory, home to a potentially combustible mix of religious groups with conflicting political loyalties, they had avoided that stark choice.

Wary of opening a new front, warring parties allowed villages to cooperate quietly with both Assad's Alawite-led forces and the majority Sunni rebels struggling to topple him.

Each handled it in its own way. Al-Zara, for instance, looked like a loyalist enclave by day: schools were open, the two-starred state flag flew overhead, and pictures of Assad were neatly hung in municipal offices.

At night, it transformed into a smuggling hub for food, medicine and weapons headed to rebels in nearby villages fighting to end four decades of Assad family rule.

"Everyone knew it was happening. The guys would even pass supplies through the checkpoints run by Assad's militias. In front of their very eyes," said Salem, a construction worker from the village of 8,000.

Now the Syrian army, emboldened by a series of battlefield victories nearby, is forcing these villages to make a choice: surrender or be crushed.

The results could be explosive. While locals here feel the army is likely to win - many minority villages are fighting to support Assad - the cost will be high.

"Anyone here with any rationality can see that the outcome of fighting here would almost certainly end in mutual destruction," said another local from al-Zara.

That may be a price the army is now willing to pay, as recent gains mean Assad's goal of securing a belt of territory between the capital Damascus and his stronghold on the Mediterranean coast is within reach.

Last month the army, led by Lebanon's Shi'ite militant group Hezbollah, seized the strategic town of Qusair near the Lebanese border. If they can cement that win by seizing this rural stretch of Homs, they may also be able to shut down rebel supply lines to central Syria and cut off rebels in the north from their comrades in the south.

"WE EAT THEM FOR LUNCH"

Large villages like al-Zara for months juggled a delicate balance between ideological sympathies and geographic realities.

As part of Syria's marginalised Sunni Muslim majority, residents identified with the Sunni-led revolt against four decades of Assad family rule.

Thousands of displaced relatives of fighters who joined the revolt have taken refuge in the once peaceful town.

But al-Zara is surrounded by villages home to Assad's own minority Alawite sect, an offshoot of Shi'ite Islam, which has largely stood behind the president.

Beyond the ring of Alawite villages is a valley dotted with Christian villages that have largely thrown their weight behind Assad. Fearing the rising tide of Islamism, they joined Alawites in creating "shabbiha", the term for pro-Assad militia.

Unlike the hostility that quickly divided Alawites and Sunnis, Christians and Sunnis tried to overlook the growing sectarian divide, recalling a long history of coexistence.

But Christian shabbiha have promised to support the new army offensive. They say the situation is now black and white.

"There are no good Sunnis. There are only the bad Sunnis, who don't carry weapons, and the worst Sunnis, who do," said a fighter from the valley who called himself Johnny.

"If we don't beat them now, they will beat us ... so I say we eat them for lunch before they have us for dinner."

Johnny's village is a few kilometres (miles) away from the hillside Sunni town of al-Hosn, famed for the majestic Crac de Chevaliers castle overlooking it. For the first time in centuries, the crusader fortress is under threat - this time from the artillery and airstrikes of army forces trying to push out rebel forces.

GHOST OF QUSAIR

The ancient site, with its strategic location overlooking the province, is now a major target for blockade or capture. Local shabbiha say Assad may even bring in fighters from Hezbollah, who played a critical role in the Qusair victory.

Qusair has served a useful tool for intimidating locals who fear a similar fate - Assad and Hezbollah forces razed much of the town when they invaded.

In the nearby border town of Telkalakh, Assad's forces reneged on a truce they made with the opposition in early 2013. According to one activist there, they called out on loudspeakers: "You saw what happened in Qusair. Surrender with your weapons and we will guarantee your safety."

Like many towns in the area, Telkalakh had operated under a deal agreed with both the rebels and Assad's forces. The army moved out its troops and checkpoints, and the rebels expelled foreign militants smuggled in from neighbouring Lebanon.

Rebels let civil servants from neighbouring towns enter Telkalakh each day to work at regional government offices, and the army controlling nearby roads let locals bring in food.

But tensions after the Qusair battle frayed relations and sparked clashes. Soon, residents said the rebels were negotiating the number of civil servants allowed inside town each day based on the amount of food let in by the army.

It was residents who pressured the rebels there to surrender this month, hoping to maintain the region's fragile peace.

Given Telkalakh's surrender, Assad's forces anticipated a quick capitulation in al-Zara when they issued a similar ultimatum this week.

Instead, they were hit with a fierce assault by rebels who had secretly been gathering in the area and decided to fight, defying the wishes of many locals.

"The Alawite villages surrounding us are ferocious about defending the regime," said Salem, the al-Zara construction worker. He is worried about the consequences of his town choosing battle. "We have nothing to gain from resistance."

By late Thursday, the rebels managed to strike at an Alawite village nearby and steal army tanks. The result, locals said, was a huge army mobilisation including air strikes heard throughout the region on Friday morning.

"THERE WILL BE BLOOD BETWEEN US"

Rebels are still trying to negotiate neutrality with neighbouring Christian towns.

A local priest in the valley, who asked not to be named, said rebels have asked him to mediate with the Christian shabbiha to prevent bloodshed.

"Rebels told me, 'What brings (Muslims and Christians) together is more than what pulls us apart. But if the Christians join this attack there will be blood between us'," he said.

He has been in talks to encourage the shabbiha not to fight. But they say it is too late, loyalties have been drawn.

"We did what was required. Al-Hosn was warned to surrender," said the fighter Johnny. "This is a war. The faint-hearted don't win wars."

The shabbiha have now begun torching the forests surrounding the hillside rebel town, to destroy any hiding places for the rebels as the army tries to impose a blockade.

Raed al-Shaer, a rebel commander in al-Hosn, warns that Christians may find similar surprises as al-Zara's Alawite neighbours did this week.

"Our fate is tied with the people of the valley," he told Reuters in an interview. "We have sleeper cells among the people of the valley. No military action has been requested of them - yet."

Fighters in al-Hosn say that unlike previous deals arranged, the choice between war and peace is now out of locals' hands.

"The choice is not with them, it was made by the rebels," one activist in the town said. "And it is: We fight, or we all die trying."

(Writing and additional reporting by Erika Solomon; editing by Philippa Fletcher)