tactic for the cartels. Eleven headless bodies were found together in the tourist resort of Merida last month, while the head of a local police commander in Acapulco was found attached to a fence outside his office in June 2006. Those fighting the cartels are in constant danger: around 450 soldiers and police officers have been killed so far this year, and the acting chief of Mexico's federal police force was assassinated in May. Judges and politicians, meanwhile, face the dilemma of plata o plomo - silver or lead: be bribed or die.

What about civilians?

In the worst-affected cities, bystanders are caught in a three-way crossfire of inter-gang fighting and government raids. Kidnapping has soared 40 per cent since 2004, giving the country the highest kidnap rate in the world. This has prompted many middle-class Mexicans, not just the super-rich, to take out ransom insurance and have micro-chips inserted in their arms to track their locations at all times. It was the kidnap and murder of 14-year-old Fernando Marti (he was killed after the ransom was paid) that led to a series of mass protests against the cartels in Mexican cities in late August.

Who is arming the gangs?

If the drugs flow north from Mexico, the guns come south from the US. As well as assault rifles and powerful handguns - mostly bought at gun shows in Texas - the gangs have machine guns, rocket-propelled grenades and shoulder-fired missiles, bought on the black market. They have even taken to using submarines: in July, a 31ft submarine was intercepted carrying six tonnes of cocaine from Colombia. The leading cartels even employ former soldiers and special forces operatives - known as sicarios ('hit men') - hired from across Latin America. The Gulf cartel, for instance, has 'The Zetas': a paramilitary group formed by deserters from Mexico's Special Forces Airmobile Group.

And how high does their influence reach?

Close ties between Mexico's drug cartels and its law-abiding establishment have existed for decades and are one of the major obstacles to defeating them. The gangs have penetrated every level of government, from local police to army generals to presidential aides. Surprise raids are impossible because, as deputy secretary of public security Patricio Arias puts it, "everyone in the world knows we're coming". In 2005, a fifth of all 7,000 agents in Mexico's Federal Investigative Agency were inspected for corruption, and the entire police force of Nuevo Laredo was suspended.

What's the US doing about this?

Calderon has gone cap in hand to Washington, asking for something similar to the $5bn 'Plan Colombia' that has weakened the cartels there. Last October, Congress approved a three-year, $1.4bn anti-drug aid package, a third of which has been distributed. But the deal has raised the question of whether this is only Mexico's problem. The country, after all, is just a conduit for America's mighty drug habit, and many now argue that the US should be doing more to control the addictions and guns that feed the current violence.