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, all from the Washington area — 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 following 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, the brother of President Hamid Karzai. The newspaper quoted sources alleging that Ahmed Wali Karzai — who denies any involvement in the drug business — collects “huge” fees from traffickers for allowing trucks loaded with drugs to cross bridges he controls in the southern part of the country.

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 agents Leamon, Michael and Weston?

Afghanistan’s status as a narco-superpower is another reason why President 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 of the country produce the raw material for an estimated 90 percent of the world’s heroin. Money from the opium 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 — and many of the Afghan citizens a counterinsurgency strategy would try to protect — are dependent on the drug trade as well. Except when the corruption that is an intrinsic element of the drug business 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.

As The Washington Post’s South America correspondent during the administration of George Bush the Elder, I watched firsthand our government’s costly and futile crusade against the cocaine industry. We tried attacking the problem in the coca fields — I visited a U.S.-financed military base in Peru’s Upper Huallaga Valley, where at the time 60 percent of the world’s coca was grown. We tried going after the processors — in Colombia, police took me to a jungle camp where chemists had been hard at work just hours earlier. We tried breaking up the trafficking cartels — I was served lunch at a Medellin prison by three cocaine bosses whose comfortable incarceration was almost like an extended stay at a hotel.

Nothing worked. All the U.S. managed to do was shift the coca fields from one valley to the next and break the big cartels into smaller ones. Profits from the drug trade still sustain a guerrilla insurgency in Colombia that has controlled huge swaths of the countryside for more than four decades. Meanwhile, cocaine is readily available throughout the United States. The illegal drug industry is driven by demand: As long as some people want drugs, other people will find ways to supply them.

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 who buy, transport and process the drugs — which creates a different kind of problem.

Those middlemen logically seek, and obtain, official protection. In Latin America, they approach police and government officials with an offer of plata o plomo — silver or lead — meaning the officials can choose to accept the bribes they are being offered, or they can choose to be shot. In a country as poor as Afghanistan, with such weak central authority, 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?

Eugene Robinson’s e-mail address is eugenerobinson(at)washpost.com.