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.

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