Since then, many thousands of people have been killed.
Source: BBC News
What is the scale of the violence?
National Intelligence chief Guillermo Valdes said on 2 August that more than 28,000 people had died in drug-related violence since 2006.
The figure was a big jump on the previous estimates of almost 25,000 released by the attorney general's office in July.
Government officials have repeatedly said that the figures need to be seen in context; they suggest that the vast majority of the killings involve people connected with the drugs trade or law enforcement officers.
Where are the worst-hit areas?
Mexico's northern border towns are experiencing the worst of the violence. Ciudad Juarez (just across from El Paso in Texas) is the city suffering the most. There are also high levels of violence in Michoacan and Guerrero states. However, Mexico is a large country, and there are still many areas where the serious crime rate is unexceptional.
The overall murder rate is lower than several other countries in the region, including El Salvador and Honduras: 11.6 murders per 100,000 inhabitants, against 51.8 and 60.9 respectively.
Does the discovery of mass graves suggest the violence is increasing?
Mass graves have been turning up increasingly frequently - some containing dozens of bodies. Beheadings and bodies hung from bridges point to a rise in gruesome attacks.
The Mexican government argues that the violence shows that the gangs are turning on one another - reflecting the success of government policies. However, some observers argue that the cartels have become so powerful that, in effect, they control some parts of the country - the violence is evidence of their gang law.
President Felipe Calderon has deployed troops. Is this strategy working?
More than 50,000 troops and federal police are actively involved in Mexico's so-called war on drugs. The Mexican government says record amounts of drugs have been seized, and senior cartel leaders jailed or killed in operations.
But another consequence has been an explosion of violence, as the drug cartels fight both the army and each other. There are also concerns about the military's lack of accountability.
How serious is corruption within the police?
Very. One reason why the government has deployed the army so extensively is that it feels the police cannot be trusted. Drug cartels with massive resources at their disposal have repeatedly managed to infiltrate the underpaid police, from the grassroots level to the very top. Efforts are under way to rebuild the entire structure of the Mexican police force, but the process is expected to take years.
How much support is there for the government's policy?
Most Mexicans support the strong line but the increasing violence has prompted calls for a rethink. Mr Calderon, while stressing he is against legalising drugs, has said he would be open to a debate on the issue.
Who are Mexico's powerful cartels?
The cartels control the trafficking of drugs from South America to the US, a business that is worth an estimated $13bn (£9bn) a year. Their power grew as the US stepped up anti-narcotics operations in the Caribbean and Florida. A US state department report estimated that as much as 90% of all cocaine consumed in the US comes via Mexico.
There are roughly seven main gangs. Alliances between them have been seen to shift as they vie for control of trafficking routes.
To what extent is violence spilling over the US-Mexico border?
Most of the violence remains firmly on the Mexican side of the border, but there is some evidence of increasingly violent attacks on US border patrol agents by drug traffickers. There has also been a reported rise in drug-related shootings and kidnappings in some US cities and towns, especially in the south-west.
A US Congress report in 2008 drew on evidence from intelligence sources suggesting that Mexican cartels had forged closer links with established drug gangs inside the US.
A May 2010 report from the US National Drug Intelligence Center said that Mexican drug trafficking organisations "continue to represent the single greatest drug trafficking threat to the United States".
What has been the US response to the drug trafficking and violence?
In March 2009, the US government announced that it would step up efforts to disrupt the illegal flow of weapons and drug profits from the US to Mexico - a key demand of the Mexican government.
The US has joined Mexico, Central American nations, Haiti and the Dominican Republic form part of the Merida Initiative - a $400m scheme to assist Mexico's efforts to take on the drugs trade, by helping to provide equipment and training to support law enforcement operations