The opium poppy was introduced to Afghanistan more than 2,300 years ago by the armies of Alexander the Great. His forces were eventually driven out, like those of every would-be conqueror since. The poppy has proved more tenacious.

On Monday, three U.S. Drug Enforcement Administration agents — Forrest Leamon, Chad Michael and Michael Weston — were killed in a helicopter crash in western Afghanistan. U.S. officials have released few details about the incident. The Times of London reported that the aircraft was shot down after a raid on the compound of a prominent Afghan drug lord.

On Wednesday, The New York Times reported that the CIA has been making regular payments to a suspected major figure in the Afghan opium trade: Ahmed Wali Karzai, brother of President Hamid Karzai. Sources allege that Ahmed Wali Karzai collects “huge” fees from traffickers for allowing trucks loaded with drugs to cross bridges he controls.

So is it our policy to attack the Afghan drug trade while we also line the pockets of one of its reputed kingpins? Who is going to explain this to the families of the dead agents?

Afghanistan’s status as a narco-superpower is another reason why President Barack Obama would be wrong to deepen U.S. involvement. Opium is the one booming sector of the Afghan economy: Poppy fields in the south and west produce the raw material for an estimated 90 percent of the world’s heroin. Money from the trade supports the resurgent Taliban, which is fighting to expel U.S. and NATO forces. Therefore, a blow against the drug business is a blow against the enemy.

Except when it isn’t. Except when the “good guys” who are supposed to be our allies are dependent on the drug trade as well. Except when the corruption that is an intrinsic element of the trade not only blurs the line between friend and foe, but also obscures the difference between right and wrong in a thick fog of moral ambiguity.

DEA officials have said they are sharply increasing the agency’s presence in Afghanistan. Wisely, the Obama administration is abandoning the George W. Bush-era strategy of trying to eradicate the poppy fields; eradication, which robs rural communities of their only livelihood, may be the quickest and surest way to turn apolitical farmers into anti-American insurgents. The focus now is on the middlemen.

Those middlemen logically seek, and obtain, official protection. In a country as poor as Afghanistan, the U.S.-backed government is vulnerable to bribery at almost every level. The inevitable future is one in which we attack and support the Afghan drug trade at the same time. Is this a policy for which we can ask DEA agents to give their lives?