On and off for over a decade, the Central Intelligence Agency conducted an audacious highly classified program to covertly manipulate Afghanistan’s lucrative poppy crop, blanketing Afghan farmers’ fields with specially modified seeds that germinated plants containing almost none of the chemicals that are refined into heroin, The Washington Post has learned.

The program’s disclosure comes as the war on narcotics is again dominating the security agenda.

President Donald Trump has declared war on drug cartels in the Western Hemisphere, ordering more than a dozen lethal strikes on alleged drug boats in the Caribbean and the eastern Pacific, designating cartels as terrorist groups, and moving a vast naval and air force to the region. He has also authorized the CIA to take aggressive covert action against drug traffickers and their supporters.

In Afghanistan in the early 2000s, the burgeoning opium trade was thwarting U.S. goals, as American troops engaged in a deadly struggle to defeat the Taliban, eliminate terrorist groups and stabilize the weak Western-backed government. Afghan heroin fueled corruption in President Hamid Karzai’s government and in the provinces. It helped pay for the Taliban’s weapons and equipment. And it accounted for the majority of global heroin supplies, with most of the drugs bound for Europe or the former Soviet Union.

Western allies and U.S. government agencies argued bitterly over which strategies would dent the crop without undermining rural Afghan support for Karzai. Diplomats and drug enforcement officials debated everything from aerial herbicide spraying to purchasing the entire Afghan crop and sending it overseas to be processed into medicine.

Unbeknownst to almost all of them, the CIA was operating its own secret heroin-eradication program, run by the spy agency’s Crime and Narcotics Center, which was flush with funds during the Afghan war. The airdrops of modified poppy seeds began in the autumn of 2004, three people familiar with the program said. The operation was paused at least once and ended about 2015, those familiar with it said.

Once the seeds were dropped, the goal was for the plants sprouting from them to cross-fertilize with native plants and become the dominant strain over time, degrading the overall crop’s potency.

The American plants not only contained virtually no morphine, but they were bred to sprout early and produce especially vivacious red flowers, making them attractive to Afghan farmers who, the CIA hoped, would harvest and replant their seeds.