Mexico’s Congress has just passed legislation that would decriminalize the possession of small quantities of illegal drugs. If President Vicente Fox signs the legislation (and despite some pressure from the U.S. government, it appears that he will), Mexico will join the ranks of the Netherlands and several other countries that have abandoned the “zero tolerance” model embraced by the United States.

Under the new law, possession of up to 25 milligrams of heroin, 5 grams of marijuana (about four joints), or 0.5 grams of cocaine (about 4 “lines”) for personal use would no longer be a criminal offense. Small quantities of other drugs, such as peyote or amphetamines, would be treated in the same fashion.

The legislation is a step in the right direction. One of the more odious features of the war on drugs is the practice of filling the jails with small-time (often recreational) users. But Mexico’s limited decriminalization measure does not get to the root of the growing incidence of drug-related corruption and violence in that country.

Both problems have reached epidemic proportions. There have been numerous cases of police personnel moonlighting as security forces for trafficking organizations. Prominent officials tasked with implementing anti-drug laws have been caught taking bribes from drug kingpins. Even the Catholic Church has acknowledged that some of the charitable contributions it takes in come from drug trafficking.

Violence connected with the illegal drug trade has been on the rise for several years in Mexico. Such cities as Tijuana and Ciudad Juarez have been especially hard hit. Nearly a thousand people perished in killings connected with the drug trade in 2005. Some Mexicans now worry that their country is spiraling down into a maelstrom of violence similar to that which afflicted Colombia during the late 1980s and the1990s.

And the violence is no longer confined to Mexico. Political officials in Arizona, New Mexico, Texas and California complain that drug-related violence is spilling over the border into their states. Mexican drug gangs operate openly in a number of southwestern U.S. cities.

Most of the corruption and violence is caused by the enormous black market premium in the illicit drug trade. The risk factor involved in defying the law means that drugs sell on the street for ten to twenty times more than they would in a legal setting. An aggressive trafficking organization can make tens–or even hundreds–of millions of dollars a year. That huge financial lure attracts those people who are most inclined to risk jail or death in a cutthroat trade–in other words, the most ruthless and violence-prone elements.

Unfortunately, Mexican leaders show no willingness to legalize the production or sale of marijuana, cocaine, heroin or other drugs to deflate the black market premium. Indeed, they argue that the new law will enable law enforcement agencies to devote more personnel and resources to suppressing trafficking. The basic prohibitionist strategy remains intact. That means the vast potential profit in the drug trade will persist–and so will the corruption and violence that is tearing Mexico’s society apart.

Decriminalizing the possession of small quantities of drugs is a modest sign of enlightenment. But Mexico (and other countries) need to abandon the entire prohibition model to produce truly meaningful benefits.