CARTAGENA, Colombia -- The contrast couldn't be more dramatic: As Colombia's president and the head of its largest guerrilla movement were putting their signatures on a historic peace deal, a 6-year-old boy was killed when he chased a soccer ball into a field and stepped on a land mine left behind during the half-century conflict.

Even as this nation celebrates the end of hostilities with the Revolutionary Armed Forces of Colombia, or FARC, the harsh reality that fueled the long conflict is settling in. From the security challenges posed by surging coca crops and dangerous criminal gangs to the difficult task of removing land mines and reintegrating guerrillas blamed for numerous atrocities, the work ahead is daunting.

The first test is getting sufficient political support to implement the lofty accords, signed Monday in this Caribbean resort city despite widespread distrust of the guerrillas. Polls show that a referendum on the deal is expected to pass Sunday. But the margin of victory is still in play and nobody expects the sort of strong turnout that would put the war-torn country firmly on the path to reconciliation.

Most of Colombia's challenges are in its vast, long-neglected countryside, where the guerrilla group long held sway and criminal activity remains rampant. The challenge is made all the more difficult by stiff opposition to the peace deal from the country's powerful former president, Alvaro Uribe, architect of the U.S.-backed military offensive that forced the rebels to the negotiating table, and a collapse in oil prices that has drained government coffers.

"The smooth part is going to end pretty abruptly," said Adam Isacson, a long-time observer of Colombia and analyst at the Washington Office on Latin America. "When you hit the second half of 2017 and you really do kind of fall off the edge of the Earth, it's going to get very confusing."

The land mine tragedy took place Monday in a plantain field in the town of Algeciras in southern Colombia, one of 700 settlements where land mines have been detected, most planted by the rebels and blamed for more than 11,000 deaths since 1990. Besides killing 6-year-old Yorman Gonzalez, whose body was riddled with shrapnel, the blast injured his friend, who is recovering in a nearby hospital.

"This saddens us and obviously tarnishes the happiness we feel over the signing of the accord," said Javier Rivera, the town's mayor.

Much of Colombia's attention has focused on the creation of special peace tribunals to judge the crimes of the rebels and state actors. For many families touched by the violence, the accord's provision sparing the rebels jail time if they confess to their crimes and surrender their weapons is an insult too far. It's also unclear to what extent the FARC will co-operate and compensate victims, as called for by the deal, following the poor example given by far-right paramilitary troops that disarmed a decade ago.

But if healing the wounds from a conflict that has left 220,000 people dead and almost 8 million homeless isn't daunting enough, there's the challenge of preventing the next outbreak of violence.

The FARC is the most powerful and best-organized illegal armed group in Colombia but it isn't the only one. Authorities say there's already evidence that criminal gangs and a smaller guerrilla group, the National Liberation Army, are moving into areas being vacated by the FARC.

They are drawn by Colombia's lucrative cocaine trade, which shows no sign of losing steam. The FARC has long demanded hefty payments from coca growers in areas they dominated in exchange for providing protection from U.S.-backed forced-eradication efforts. The drug profits strengthened the group even as other leftist insurgencies in Latin America were defeated.

As part of the peace deal, FARC leaders have committed to assisting the government in alternative development and eradication. But there's fear that some rebels, especially mid-level commanders who have no real political future, will resist abandoning the cash cow. And even if they do, others are willing to step in.

Getting it right matters beyond Colombia's borders. The world's largest supplier of cocaine, the country saw six straight years of declining or steady production reversed when the amount of land under coca cultivation jumped by 39 per cent in 2014 and by another 42 per cent last year, to 392,000 acres (159,000 hectares), according to U.S. government figures.

For those among the FARC's estimated 7,000 fighters who do disarm, the journey back to gainful employment and acceptance by society is likely to be a long one. The experience following the defeat of insurgencies in neighbouring Central America is a cautionary tale: many former rebels, whose only marketable skill was firing an assault rifle, ended up joining criminal gangs.

Despite the many hurdles and uncertainties, Colombians for now can hold their head high, says Michael Shifter, president of the Washington-based Inter-American Dialogue.

"The fact that the military and FARC will no longer be killing each other is a big deal. It opens up a real opportunity to transform Colombia," said Shifter, who was present at Monday's signing ceremony. "Whether the country will rise to the occasion and take advantage of the opportunity remains to be seen."