In Iraq, we need troops. In Afghanistan, we need peace and economic revival. The solution (inspired, in part, by the career of Darius the Great*) is to (1) recruit the Afghan militia-men into an auxiliary force for the occupation of Iraq, while (2) legalizing drugs in the United States.
The Afghans are almost the ideal allies for our Mesopotamian expedition: they're highly experienced in guerrilla warfare, and they're Muslims --- mostly Sunni, even. This will reduce the likely hostility that would meet, e.g., Indian or Korean troops (assuming we could get any). At the same time, they're not likely to be sympathetic to Arab nationalism, particularly not the Baathist variety. Generous pay and support, by Afghan standards, would be extremely cheap by American ones. There would be, admittedly, some danger of them simply trying to take over Iraq on their own account, but I think most of them would prefer to head back to Afghanistan eventually, especially if the right rewards were offered for their return after victory.
In Afghanistan itself, the removal of hundreds of thousands of violent armed men will do a great deal to bring peace and stability, but the country still needs a sound economic basis. The prosperity (not to say the ability to feed itself) of an agrarian population is rather reduced when much of the farm-land is seeded with landmines and irrigation has been systematically destroyed. The Afghan solution, of course, has been to grow opium poppies, so that a small amount of land can produce enough cash to buy food from abroad. (The arrangements are a bit more complicated, but that's the essence of what it comes to.) Now the problem with drugs, considered as a source of export revenue, is that they're illegal in the destination countries. This means that the business is unusually risky, and profits are often large but fluctuating. This attracts risk-loving individuals. Since the business is illegal, law cannot be used to enforce contracts or settle disputes, so violence must serve, which further attracts not only the risk-loving but also the violence-loving, and pretty soon you've got a situation where people who like to gamble with other people's lives can make a lot of money, which is bad for basically everybody else, and encourages rebellion.
Legalization turns drugs into mere cash crops, not particularly suitable for financing violent undertakings, nor particularly attractive to the violence-prone and risk-loving. (Consider coffee.) Profits can be (straightforwardly) invested, rather than blown on guns, ridiculous cars and loose women, setting the foundation for export-led growth. More money might actually reach the farmers, since more normal mechanisms of economic competition will come into play. I have no idea of the comparative quality of Afghan opium, but the hash used to be famous, and I have every confidence the country could produce an internationally-competitive product.
Domestically, of course, the benefits of legalization would be manifold. In addition to all the usual arguments about the expense of prisons, victimless crimes, respect for the law, etc., which I won't bother to rehearse, we would be able to redirect our limited law-enforcement and border-control resources away from drug control and towards counter-terrorism. This would be hard on the proverbial U.S. Customs Coast Guard dope-dogs, but they can always be reconditioned as bomb-dogs. We would also reduce the number of people with experience at smuggling things into the US, no questions asked, and the frequency with which, e.g., officers of the customs service will be tempted to ask no questions. Legalization is really an anti-terrorist measure.
Globally, we get benefits from this all over the place. Europeans not infrequently complain of our hypocritical puritanism: this would address a large part of it. We'd gain much-needed points in South America (see "Bolivia, cultivation of coca, recent unpleasantness over" ; "Columbia, forty-year-long civil war, financed through cocaine" ; etc.). Best of all, the fact of, say, 150,000 Afghans serving under the Stars and Stripes would basically annihilate the whole "clash of civilizations" nonsense that does us so much harm. People quote Kipling's "Ballad of East and West", but they don't read it, or they'd know it's about recruiting Pashtuns into the service of the British Empire, and they'd remember how it ends:
Oh, East is East, and West is West, and never the two shall meet,A few years of Americans and Afghans fighting together, especially if they're encouraged to talk about "Women and Horses and Power and War" together, will do wonders for inter-civilizational relations...
Till Earth and Sky stand presently at God’s great Judgment Seat;
But there is neither East nor West, Border, nor Breed, nor Birth,
When two strong men stand face to face, tho’ they come from the ends of the earth.
A final advantage to this plan is that, when we invade Saudi Arabia with an Iraqi army, the precedents will already be well-established.
Update, 10 August 2005: the Afghan-legion part of this idea has now (independently) been put forward as a serious proposal.
[*: F. L. Holt, Alexander the Great and Bactria (Leiden: E. J. Brill, 1988), pp. 39ff.]