YOKY RIDGE, Colombia—On a hilltop base shielded with sandbags, police sharpshooter Jose Diaz gazed into thick jungle as a fellow commando checked tripwires protecting the stronghold. A radioman listened in on the fighters they were battling.

“They’re always looking for the right moment to attack our base,” said Hector Ocampo, commander of the Colombian detachment in a cocaine-trafficking corridor near Panama.

Their adversaries weren’t the FARC rebels that security forces had long fought, but a cocaine-trafficking gang known as the Gulf Clan. In the year since the powerful Marxist guerrillas disarmed, drug gangs like this one have battled each other and the state for control of the booming cocaine trade in remote regions where the FARC once ruled.

With the gangs fighting over the spoils, homicides in drug-crop regions jumped by 45% in the first four months of this year, the Bogota-based policy group Ideas for Peace said in a recent report. Particularly alarming to many Colombians have been the killings of 181 community leaders in these regions since January 2016, many of whom had openly opposed drug trafficking, according to data from the Attorney General’s office released July 9.

“You had social order with the FARC, and then you break that order,” said Maria Victoria Llorente, director of Ideas for Peace. “And the state is unable to impose a new order.”