Opiate painkilling drugs are in critically short supply across the developing world. So why doesn't the USA just buy the Afghan poppy harvest, process it into painkilling meds, and distribute them to poor countries?
1. This would cut off the Taliban's chief source of funding.
2. It would put the average Afghan farmer on the side of the US-supported government instead of the Taliban.
3. It would play to our strength -- money. The Taliban has more local knowledge, more time, more patience, more willingess to shed innocent blood. But we have more money. They might be able to outfight us, out-corrupt us, or out-terrorize us, but they can't outbid us.
4. It would put the US on record as alleviating suffering all over the world.
5. By fighting the Taliban with dollars instead of (as many) soldiers, we'd suffer a lot fewer killed and maimed Americans.
6. It might even be cheaper. Soldiers, tanks, planes, humvees, night vision goggles, bullets, Predator drones, etc. are really, really, expensive.
The brilliant thing about this is that it doesn't require Afghani farmers, who really don't need any extra trouble, to change anything about their farming practices; it provides a market for their product that doesn't fund the Taliban or lead to horrific human rights abuses within the structure of the transaction; it's a badly needed source of revenue.
However, it's clearly not that simple. First of all, you can't just buy the opium crop and remove--or even significantly reduce--the army. You need both. If the Taliban can't rely on opium revenues, in the absence of obstruction they'll just find another way to force the population to support them. Not everyone in Afghanistan is a poppy farmer, by a long shot, and clearly the Taliban have managed them before. By the same token, Afghani poppy farmers don't sell to Taliban-connected drug lords because they pay the highest--it's because these guys have incredibly harsh protection rackets going. This is a frequent reason girls set themselves on fire at a young age--sometimes their fathers can't deliver what they need to or they find themselves "owing" a drug lord in some way, and they end up giving them their daughters in payment.
If a higher bidder just comes along, the farmers aren't going to merrily switch business partners hey presto. They would need significant protection from retribution. So, yes, you need both security forces and poppy-buying. Keeping in mind that our forces there are already deemed insufficient, I don't think we could really draw down much (although if we get out of Iraq and had poppy revenue, our presence there would be less of a strain--for the budget and for the soldiers themselves, who hopefully could go on fewer and shorter tours of duty).
The other massive problem is the likelihood that a number of soldiers would suddenly turn into drug dealers. Let's face it, our armed forces have been taking whoever they can get lately (except gay people), including convicted felons, abusers, rapists, etc. (This is not to say that all soldiers are in this mold--far from it.) It is not too much of a stretch to imagine that if you get a bunch of burned-out Marines and whatnot together in a war zone and give them a bunch of opium, they'll either start taking it or, in a region riddled with connections, start dealing it. Or, that time-honored combination, both. Really, the last thing we need is the U.S. Armed Forces basically acting as the best, most secure drug convoy in the world.
Imagine the possibilities. The opiates have to get all the way from Afghanistan to somewhere where they can be made into something useful, probably in the West, and they'll change hands several times. Then, once they're drugs--drugs which can still get you high--they have to get all the way to Sub-Saharan Africa or similar. The second stage is no more risky than humanitarian medical aid already is. But oh, god, I can see the reports coming in now and they're not pretty. The second a U.S. soldier kills someone over drugs we are screwed. Plus, once the VA has to start implementing detox programs for soldiers coming home whose families are freaked out by their withdrawal symptoms, the Armed Forces start to look pretty damn bad--especially to parts of the population who have historically had strong respect and support for the military. I mean, if anything says THE OPPOSITE OF MILITARY DISCIPLINE AND HONOR in big red letters, it's a smack problem.
So I guess my point is that the above proposal is an admirable solution to several real-world issues that have been bungled or not addressed up to now; it just ignores a whole host of other real-world issues that would accompany it. That's the thing with policy: it always does a million things other than the things you designed it to do.