The war in neighboring Syria has had devastating consequences on Lebanon's economy — with a notable exception. Pot growers in the Bekaa Valley, near the border, have seen their profits soar over the last two years.

The country's military has been so thinly stretched in trying to prevent the conflict next door from spilling over that no resources were left to carry out the eradication campaigns of previous years and cannabis cultivation has boomed.

But as Islamist fighters from both the Islamic State and Jabhat al-Nusra push closer and sometimes into Lebanon, weed farmers that have occasionally taken up arms against their own military are now swearing they'll do the same to fend off anyone else bent on destroying their crops.

Islamic State militants, in particular, who have imposed strict sharia law in the caliphate they proclaim in parts of Syria and Iraq, consider cannabis to be un-Islamic and have publicized their efforts to torch the illegal crops — as well as other products they consider haram, or forbidden under Islamic law, like alcohol and cigarettes.

So far, the Islamic State has not traded its principles for the sake of pocketing drug money, but how long they'll last is in question, Vanda Felbab-Brown, a senior fellow at the Brookings Institute and expert on counter-narcotic efforts in conflict zones, told VICE News.

Islamist militants around the world — from Mokhtar Belmokhtar in North Africa, to Hezbollah in Lebanon, and the Taliban in Afghanistan — have embraced the illicit drug trade, "which clearly is inconsistent with Islamic teachings but very much justified because of the fight," Felbab-Brown said. "Most militant groups become very pragmatic and in my mind the Islamic State will evolve on that issue."

"One of the things that's interesting about the Islamic State is that they are taking this position because many militant groups around the world, including Hezbollah, learned that the anti-drug policy was very counterproductive. The Taliban is the classic example," she said. "And of course the Islamic State has been very involved in a variety of illegal rackets, and how long they will maintain this policy is a big question."

But whether against the national military or Islamist militants, Lebanon's pot growers are ready to defend their fields. And they are armed to the teeth.

"I'll burn a tank if I hit it with it. If I hit a pick-up with Islamic State people in it, it will burn," Abo Hamoudi, a 65-year-old grower recently told Australian network SBS, showing off a rocket-propelled grenade. "If I hit any vehicle with Islamic State in it, I'll burn it."

Hamoudi is not alone. Well-armed cultivators, who have engaged in turf battles against each other and the Lebanese army, have pledged to turn those weapons on any fighters coming into Lebanon from Syria.

Lebanese authorities, for their part, want the cannabis farms gone, but they could also use the help against the militants.

"I think they're using Islamic State militants as an excuse to justify having weapons but the real reason is to protect their hashish," Lebanon's chief of drugs enforcement, Colonel Chassan Chamseddin, told SBS. "But of course if there's any assault from outside of Lebanon into Lebanon they may use their weapons to help the army."

Lebanon is known among drug connoisseurs for producing some of the world's finest and most sought after cannabis products, including the famous Lebanese blonde and red varieties of hashish.

Hezbollah, which wields much influence on the Bekaa Valley where many of the cannabis farms are located, has long controlled the illicit trade that sustains much of the area's economy.

"You have the constellation of military forces being preoccupied elsewhere," Felbab-Brown said. "But also the fact that Hezbollah has more political power and can prevent eradication, and very much understands that the Bekaa Valley is a core part of its constituency and there is a socio-economic benefit to cultivating cannabis."

"In a way it's analogous to the Taliban opposing poppy eradication in Afghanistan," she added. "They may oppose use of the drugs on a religious basis, but they see the economic benefits of producing it, and they'll say 'you cannot consume it, that's wrong, but as long as it goes to the kafir it's fine.'"

The Lebanese military had been raiding the farms since the end of the country's civil war, in 1990, with significant funding from the US, bulldozing or burning down crops and sometimes engaging in open conflict with growers, who responded to the raids with assault rifles, mortars, and rocket-propelled grenades.

But the eradication campaign came to an end in 2012 because of growing instability on the Syria-Lebanon border — prompting growers to double down on cultivation.

In 2013, production grew so fast that prices for a "bundle" of Lebanese red (about six ounces) went down from $50 to $30, the Daily Beast reported.

"Every year we are increasing the areas we are planting," Ali Nasri Shamas, a pot farmer, told Voice of America earlier this year, challenging the Lebanese authorities. "Three years ago, we told them we will plant double. We did, and we will confront them. The next year, we promised them we would plant five times that amount. We did and we confronted them. And we will increase it every year. Either they provide an alternative, they legalize it or it will be a confrontation between us and them."

Authorities calling for crop destruction have promised farmers compensation and alternative sources of income — pledges that have yet to materialize.

But with Lebanon's tourism industry and its legal crop exports taking huge losses because of the four-year Syrian war, cannabis cultivation remains one of few sustainable sources of income for local residents.

In the early 1990s, when a UN-sponsored eradication program first started, nearly 30,000 hectares of cannabis was destroyed, leaving 250,000 people and 23,000 family farms without a source of income, according to Sensi Seeds, a cannabis seed "bank" and global dealer. Today, with Lebanon's economy on a plunge, the illicit economy is often the only one available.

Calls for Lebanon to legalize cannabis are growing — with farmers and economists alike arguing it would make sense. Economist Marwan Iskander estimated that legalizing the crop would inject some $400 million to the country's budget and $2 billion into its wider economy. And Noah Zaiter, the leader of one the country's most powerful drug families, once famously said, "make marijuana and hashish legal for six months and I'll pay down all government debt" — around $36 billion.

"The Bekaa Valley is one of the poorest areas in Lebanon and it's very much been the core base for Hezbollah for a very long time, precisely because it's so economically underdeveloped compared to other parts of the country," Felbab-Brown said. "But the biggest problem for Lebanon is the constant political instability. It's a country that's extremely politically unstable, extremely ethnically fragile, and always hovering on the brink of war, whether with outside countries or civil war."

That's as ideal for the illicit drug trade to flourish as the fertile ground of the Bekaa Valley.

Lebanese authorities and foreign militants are warned. As Shamas told local reporters, Lebanon's pot growers "will break the hands and legs of anyone who dares destroy our crops."

Follow Alice Speri on Twitter: @alicesperi