Here's Alfred McCoy on ILLEGAL DRUGS:
Mike Levine said that he wanted to go up country to Chiangmai, the heroin capital of Southeast Asia at that point, the finance and processing center and hub of an enterprise. He wanted to make some major seizures. Through a veiled series of cut outs in the U.S. embassy in Bangkok, instructions were passed to his superiors in the DEA, who told him he couldn't go up and make the bust. He was pulled off the case.
He said it wasn't until he read my book a number of years later that he understood the politics of what was going on and he realized why he had been pulled off. All of the upland drug lords that were producing the narcotic, the heroin, were in fact CIA assets. Now he understands it.