There have been a tragic 60,000 killings under President Felipe Calderón that are described as drug-related. But even this description can be misleading. Most cartel assassins do not carry out these brutal acts because they are high on drugs. Their motive is to capture the profits that are so high because in the black market you can buy drugs for a nickel and sell them for a dollar. How many others would love to be in a business with a markup of more than 2,000 percent?

Marijuana is just one of the drugs that the cartels traffic. Chemicals such as crystal meth may be too venomous to ever be legalized. But cannabis is a cash crop that provides huge profits to criminal armies, paying for assassins and guns south of the Rio Grande. The scale of the Mexican marijuana business was illustrated by a mammoth 120-hectare plantation busted last year in Baja California. It had a sophisticated irrigation system, sleeping quarters for 60 workers and could produce 120 metric tons of cannabis per harvest.

Again, nobody knows exactly how much the whole Mexico-U.S. marijuana trade is worth, with estimates ranging from $2 billion to $20 billion annually. But even if you believe the lowest numbers, legal marijuana would take billions of dollars a year away from organized crime. This would inflict more financial damage than soldiers or drug agents have managed in years and substantially weaken cartels.

It is also argued that Mexican gangsters have expanded to a portfolio of crimes that includes kidnapping, extortion, human smuggling and theft from oil pipelines. This is a terrifying truth. But this does not take away from the fact that the marijuana trade provides the crime groups with major resources. That they are committing crimes such as kidnapping, which have a horrific effect on innocent people, makes cutting off their financing all the more urgent.

The cartels will not disappear overnight. U.S. agents and the Mexican police need to continue battling hit squads that wield rocket-propelled grenades and belt-driven machine guns. Killers who hack off heads still have to be locked away. Mexico needs to clean up corruption among the police and build a valid justice system. And young men in the barrios have to be given a better option than signing up as killers.