Elite Colombian forces had been hunting the commander, Alfonso Cano, 63, for three years since he ascended to the top of the Revolutionary Armed Forces of Colombia, or FARC, following the death in 2008 from a heart attack of Manuel Marulanda, the guerrillas’ legendary leader. Earlier that same year, Colombian forces killed FARC’s second-in-command, Raúl Reyes, in a raid carried out on Ecuadorean soil.

The setbacks suffered by FARC, which also included the killing last year of its field marshal known as Mono Jojoy, form part of a broad weakening of the rebel group during the past decade, with hundreds of its combatants deserting in recent years and its ranks thinning considerably from a peak of an estimated 17,000…

The killing of Mr. Cano in Cauca, a region in southwest Colombia, allowed some in the country to ponder whether FARC was finally being marginalized as a security threat. “No one else can keep the group together like he did,” said Marta Lucia Ramírez, a former Colombian defense minister, speaking of Mr. Cano.

“They’ve stopped being a threat for Colombian democracy,” said Ms. Ramírez, “but they continue being a threat to the citizenry.”