The largest single change for the better in U.S. foreign policy, and one that could be accomplished simply by an act of political will, would be the abandonment of the so-called War on Drugs. This last relic of the Nixon era has long been a laughingstock within the borders of the United States itself (where narcotics are freely available to anybody who wants them and where the only guarantee is that all the money goes straight into criminal hands). But the same diminishing returns are now having a deplorable effect on America’s international efforts.

Consider the case of Afghanistan. Thirty years ago, it was a vine-growing country, renowned for its raisins. It is now so deforested that a farmer planting a vine would be an optimist, while a farmer growing poppies is assured of at least some income. We burn and destroy what is in effect the Afghans’ only crop, while suffering from a shortage of analgesics in the United States. The beneficiaries of this policy are the Taliban. Why not instead buy the Afghan crop, use it to manufacture painkillers, and burn or throw away the rest (if you insist) while simultaneously offering incentives and aid to vine growers? We already pay the Turks to grow medical opium; they don’t need the money. The revenue that now goes to drug lords and terrorists could be applied straight to Afghanistan’s reconstruction, while weakening those who benefit from an artificially created monopoly. This might be termed “win-win.” And this is to speak only of opiates. The usefulness of marijuana in combating glaucoma and in helping to ease the pain of chemotherapy is now well attested.

Decriminalization of drugs could also mean fewer lethal impurities (the result of gangsters “cutting” the stuff) and a decline in the glamour associated with prohibition. The opportunities for the corruption of...