Vicente Fox, Mexico's president, has just performed a sudden and remarkable U-turn on drugs policy. Barely a day after his office announced that he would sign into law a measure, already passed by the Mexican congress, decriminalising the possession of small amounts of hard drugs such as cocaine and heroin, Mr Fox backed down.

The law would have given Mexico probably the most liberal drugs consumption regime anywhere, more permissive even than Dutch laws. It provoked near hysteria among US officials, who forecast floods of young Americans crossing the border to this new nirvana of drugs tourism. But the aborted measure speaks volumes about Mexico's impossible position in the chain of drugs production and consumption - created largely by Americans' appetite for narcotics.