The Tierra Grata encampment in the foothills of the Perijá mountains overlooks vast cattle ranches around the city of Valledupar. If the FARC guerrillas were still waging war on the government of Colombia, it would be the perfect spot from which to dominate this northeastern area.

But the 160 members of the FARC's 41st and 19th fronts who occupy the hillside camp spent a recent Sunday preparing not for battle but for a soccer tournament with teams from nearby towns. They are among nearly 7,000 guerrillas in 26 camps across the country who are waiting to disarm and become civilians under a peace deal, ratified last December, that ends the group's 52-year-long war against the state.

But even as the FARC players warmed up, there were signs that not everything was going to plan. The camp is still under construction, which should have finished last year. The FARC's ammunition and 7,000 firearms should have been deposited in shipping containers secured by the U.N. by the end of April.

But by May 5, the U.N. had collected just 1,000 weapons. It has asked for extra time to take control of 900 caches of arms and explosives hidden in jungles and mountains. Colombia's president, Juan Manuel Santos, is considering the request. "One more month, or six more months — after a war of 52 years, is it really important?" he asked.

The biggest deadline is the end of May, which Colombians call "D plus 180" because that many days have passed since the accord took effect. By then the FARC should cease to exist as an armed group, and its adherents should be certified as civilians and thus free to leave the camps. All that is unlikely to happen on time.

The missed deadlines are a warning signal for a peace agreement that still faces formidable critics. The opposition, led by Álvaro Uribe, a former president, remains implacable in its hostility.

Voters elect a new congress next March and a new president in May. The FARC are supposed to take part: they have been guaranteed five seats in the Senate and five in the lower house. Santos, who cannot run again, must prove to voters by then that the peace process is working.

So far, the FARC and the government are not allowing delays to endanger the peace.

Nonetheless, FARC's leaders fear for the future. Although the peace accord requires the government to provide protection, in April two FARC guerrillas and five relatives of FARC members were slain in four separate incidents, said CERAC, a group that monitors conflict. The government acknowledges that during this year 14 social leaders who backed the peace deal or defied organized-crime groups have been killed. Human rights groups say the number is far higher.

Congress has passed a law granting amnesty to FARC fighters, who have committed crimes ranging from drug-trafficking to murder. But courts have delayed releasing from prisons 2,000 FARC members.

The FARC's biggest worry, though, is that the next government will be less committed to the peace agreement than the current one is. Uribe and his supporters say the accord does not punish the FARC sufficiently. Fernando Londoño Hoyos, a leader of Uribe's Democratic Centre Party, said his party will rip the agreement "to shreds" if it takes control of Congress and the presidency.

Seeing that danger, Santos pushed through the legislature a constitutional reform that would make that impossible. It obliges the next three presidents to uphold the letter and spirit of the accords. But the constitutional court has two months to decide whether the language passes muster.

These uncertainties have made the FARC slow to surrender their weapons. That feeds unease among Colombians, who already detest the FARC. Many fear that the peace deal is encouraging new forms of lawlessness. Coca-growing has surged; farmers are planting the shrub in order to pocket payments for ripping it up again.

Colombian officials say it is still possible that the FARC will hand over their firearms by the D-plus-180 deadline. If that happens, the government can claim that the FARC at long last have ceased to be a threat. And the FARC can then turn their attention to winning with ballots the power they failed to seize with bullets.