Last week Colombian and FARC leaders shook hands in Havana. It was the first tangible promise of peace in a half century. But despite this, 170,000 acres of an emerald-green plant still threatens violence.

The cease-fire between the Colombian government and the Revolutionary Armed Forces of Colombia (FARC) ended a 50-year war that killed 250,000 people, and displaced 6 million, a population twice the size of Chicago. Barring the unpredictable, the 8,000 FARC soldiers who have lived and killed in the jungle will soon put down their weapons and relinquish control of 60 percent of the world’s most-productive coca crops.

It’s unclear what exactly will happen to the coca fields when FARC leaves. The government hopes the Marxist guerrilla group will help farmers transition from coca to another crop, leading to a tranquility that’s been absent from rural Colombia since bands of poor farmers fled to the mountains in 1948. But FARC’s absence could also create a vacuum that any number of other criminal groups already in the jungle may seek to fill, giving rise to more violence.

FARC operates in 25 of Colombia’s 32 provinces. When it’s time to demobilize, its soldiers will come to designated safety zones where they will surrender their  guns, and begin the government-supervised process of reintegration, something that will be tough for the individual guerrilla.

“It implies a whole different life for us, a different regime,” Alberto Camacho, a FARC commander, told The Guardian. “We have been living in boots for so long.”

It will also be tough for FARC to extricate itself from the cocaine business, which has earned it anywhere between $200 million and $3.5 billion a year.

FARC has never admitted its full involvement in the drugs trade. It began as a political movement, when a group of communist farmers ran to the mountains to protect themselves from the government. In the 1950s, nearly all of Colombia had split into armed political factions, and the government saw the farmers’ Marxist ideology as a threat. The farmers fought back and their action gave birth to FARC and an armed rebellion. At its peak, FARC claimed 20,000 fighters. And it owes its success and survival to the money brought in through kidnapping, extortion, and beginning in the early 1980s, cocaine.

Although growing coca in Colombia is illegal, poor farmers cultivated it because by the time they transported their legal crops and fruits to the markets from their remote towns and villages through the dense forests, the transportations costs eroded any profit margin.

“But with coca,” Jeremy McDermott, co-director of Insight Crime, told me, “the buyer will come to your house.”

Two years ago, McDermott, who lives in Medellin, wrote about FARC’s ties to cocaine in an article titled “The FARC and the Drug Trade: Siamese Twins?” In it, he wrote FARC taxes farmers $50 per kilo of cocaine and criminal organizations $200 per kilo. It charges $100 per kilo to the laboratories that soak the green leaves in kerosene to extract the alkaloids that 1.5 million U.S. cocaine users regularly snort. FARC charges airplanes that transports the drugs from its territory, and has even shipped cocaine itself.

FARC soldiers work in units called fronts, which are its fighting blocs. Each is responsible for itself, but answers to the Secretariat, the rebel group’s governing body. The jungle is vast and the guerrillas use the small towns spread in the countryside as bases of operation. For that reason, and because of the nature of being a rebel group hiding from the Colombian government, FARC’s central command can’t oversee everything. One fear is that after peace, autonomous fronts will stay put in the jungle, having forged their own relationships with mafia and cartels, completely able to oversee the growth, manufacture, and shipment of cocaine.

“That is the ‘FACRIM’ scenario,” McDermott told me.

That term is something McDermott coined based on another armed rebel force that disbanded and turned into fractured criminal groups. They are now collectively called BACRIM, a combination of “bandas criminales,” Spanish for criminal bands. Most of the BACRIM began as the United Self-Defense Forces of Colombia (AUC), a right-wing group supported by both drug dealers and the Colombian army. For a while, the government found AUC useful. In the 1990s and early 2000s, it hunted and killed FARC members. But the U.S. later labeled it a terrorist organization—like FARC—and by 2006 it had largely demobilized. Some of its fighters remained in the jungle, and these groups now buy cocaine from FARC-controlled growers.

Another possibility is that FARC soldiers with a stronger desire to hold onto their fields rather than make peace could take their coca-knowledge and their holdings to the National Liberation Army (ELN), another communist guerrilla group with a similar history. The ELN was founded in the 1960s and headed for a long time by leftist Roman Catholic priests. It’s now a designated terrorist organization with ties to drug smugglers. Although ideologically similar,  ELN and FARC have not always gotten along, but they formed a loose alliance after 2008.

Coca farmers have been able to grow in relative comfort because of FARC’s protection, in exchange for a fee. If FARC leaves, ELN could slip into this role, bolstered by FARC’s dissident blocs. The ELN is also in peace talks with the government, although they are not as far along. This could mean the ELN also bows out of the cocaine business, leaving it to others. And that could lead to the scenario with the greatest likelihood for violence, which is one where no one clearly controls the cocaine.

Convincing farmers to grow bananas instead of coca is an entirely different battle. Those who’ve tried to make the switch often find they earn one-third of what they did growing coca. The government says it plans to subsidize these farmers, weaning them onto another crop. That will take time. And when the FARC drops its weapons, it will leave behind a desirable void, a vacuum of control worth billions.

“How do you fill the vacuum in these areas where the criminal economy makes up 70 percent of the whole economy?,” McDermott asked.

That’s what the Colombian government is trying to figure out now that the most stabilizing force in those areas, FARC, is preparing to leave. The most optimistic future is one in which FARC uses its influence to ease farmers into legitimate crops. But it will be difficult: In 2014 Colombians planted 44 percent more coca plants than the year before. Last year, they likely planted even more. Cocaine has been big business for decades. Then again, it was just nine years ago that FARC doubled down on attacks against the government. Now there’s peace for the first time in a half-century, so it seems anything is possible.