JUAN MANUEL SANTOS, Colombia’s president, could be a candidate for the Nobel peace prize. But a few days after signing a ceasefire agreement with the FARC, effectively ending Colombia’s 52-year-war against the guerrilla group, he is eager to talk about his military credentials. He joined the navy at 16, helped lead a military campaign against the FARC as defence minister in the late 2000s and in 2011 ordered a raid that killed their top commander. “No Colombian has hit the FARC harder than I have,” insists Mr Santos in an interview at Casa de Nariño, the presidential palace in Bogotá.

The reason for this tough talk is that many Colombians are sceptical of the deal he signed in Havana on June 23rd with the FARC’s “maximum leader”, Rodrigo Londoño, known as Timochenko. The FARC have massacred Colombians, kidnapped them for ransom, sold cocaine on a grand scale and committed other crimes in the course of a war in which perhaps 220,000 people died (though there are no reliable figures). Guerrillas who confess will be subject to eight years of “restrictions on liberty” (not jail) and community service. That is not punishment enough, many Colombians believe. Álvaro Uribe, who was president when Mr Santos was defence minister and now leads the opposition to him, accuses his former protégé of “wounding” the concept of peace.

The popular mood matters. After a final peace deal is signed, probably this summer, it will be put to a referendum. Polls suggest that “yes” will win. But if the margin of victory is thin, Mr Santos will have difficulty putting into practice the policies required to implement the accord. The government must undertake expensive rural-development programmes; low oil prices and weak economic growth have reduced the revenues needed to pay for them. Mr Santos, whose approval rating after four years of daily dealings with the FARC is a dismal 21%, will have to court further unpopularity by raising taxes.

That makes it all the more vital for Mr Santos to persuade Colombians that the peace is a just one. He has a strong case. The latest agreement sets out details of the FARC’s demobilisation to 23 rural zones and the surrender of their weapons. By assenting to ratification by plebiscite rather than by a constituent assembly to rewrite the constitution, the FARC have recognised the legitimacy of Colombian democracy and the rule of law. The peace deal will mark the first time in any country that demobilised guerrilla commanders have agreed to be investigated and punished. “There is no impunity,” Mr Santos insists.

He is not a natural salesman. Though he comes from a prominent political family, which founded El Tiempo, Colombia’s largest newspaper, he is not a gifted communicator. A friend described a younger Mr Santos as a “cyborg”, programmed in childhood to become president. Neither telegenic nor eloquent in public, he seems more comfortable among bankers than peasants. He often stumbles when explaining to Colombians how peace can transform their lives.

A reputation for slipperiness compounds the problem. Mr Santos, who fixes his own political position in “the extreme centre”, headed ministries in both Conservative and Liberal governments. He campaigned for the presidency in 2010 as a hardliner on security, then enraged many voters by opening talks with the FARC.