Thus opium has moved from Turkey and Thailand to Myanmar and southern Afghanistan, where it undermines the West's efforts to defeat the Taliban. At best, effective repression merely forces it to shift production sites. On the other hand, there is plenty of evidence that the drug business quickly adapts to market disruption. But it is not clear that drug demand drops when prices rise. The street price in the United States does seem to have risen, and the purity seems to have fallen, over the past year. Nowadays the drug warriors claim to seize close to half of all the cocaine that is produced. Even if dumping weedkiller on the crops of peasant farmers quadruples the local price of coca leaves, this tends to have little impact on the street price, which is set mainly by the risk of getting cocaine into Europe or the United States. Take cocaine: the mark-up between coca field and consumer is more than a hundredfold. The price of an illegal substance is determined more by the cost of distribution than of production.

Yet prohibition itself vitiates the efforts of the drug warriors. This week yet another leader of a troubled drug-ridden country-Guinea Bissau-was assassinated. In Mexico more than 800 policemen and soldiers have been killed since December 2006 (and the annual overall death toll is running at over 6,000). In the developing world blood is being shed at an astonishing rate. It arrests 1.5m of its citizens each year for drug offences, locking up half a million of them tougher drug laws are the main reason why one in five black American men spend some time behind bars. The United States alone spends some $40 billion each year on trying to eliminate the supply of drugs. Consumption of cocaine has declined gradually in the United States from its peak in the early 1980s, but the path is uneven (it remains higher than in the mid-1990s), and it is rising in many places, including Europe. (Like most purported drug facts, this one is just an educated guess: evidential rigour is another casualty of illegality.) The production of cocaine and opium is probably about the same as it was a decade ago that of cannabis is higher. Its boast is that the drug market has “stabilised”, meaning that more than 200m people, or almost 5% of the world's adult population, still take illegal drugs-roughly the same proportion as a decade ago. Nowadays the UN Office on Drugs and Crime no longer talks about a drug-free world. As we outline below, many vulnerable drug-takers would suffer. Legalisation, though clearly better for producer countries, would bring (different) risks to consumer countries. That is why The Economist continues to believe that the least bad policy is to legalise drugs. By any sensible measure, this 100-year struggle has been illiberal, murderous and pointless. In fact the war on drugs has been a disaster, creating failed states in the developing world even as addiction has flourished in the rich world. Like first-world-war generals, many will claim that all that is needed is more of the same. Next week ministers from around the world gather in Vienna to set international drug policy for the next decade.

Yet it is a hugely irresponsible promise, because it cannot be fulfilled. It is intended to reassure the parents of teenagers across the world. It assuages the sense of moral panic that has been the handmaiden of prohibition for a century. That is the kind of promise politicians love to make.

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