In the 1990s, the cartels cooperated with the PAN to crack down on rackets and snatch votes from the PRI.[7][8] Indeed, in 1994, a presidential candidate and PRI's secretary general was assassinated.[9][10][11] Furthermore, in 1995, President Carlos Salinas's brother, also a PRI member, was arrested for murder and corruption.[12] In this climate of corruption and political violence, the PAN won the 2000 elections with Vicente Fox.[13]Supported by the PAN, the cartels were able to focus on drugs and commission assassinations from external armed groups, such as the Zetas, former Mexican special forces soldiers who became hitmen for the Gulf Cartel. [14][15][16]
The dispute over Nuevo Laredo’s border
Osiel Cárdenas' extradition to the United States from Mexico.
The tension between Sinaloa Cartel and Gulf Cartel started in 2003 with the arrest of Osiel Cardenas Guillen, altering the previous balance of power between the cartels.[17] The Sinaloa Cartel also started challenging Gulf Cartel’s dominance of the city of Nuevo Laredo for its important drug smuggling route into the United States with their own militia: Los Negros.[18][19] According to the DEA, half of the cocaine arriving in the United States was trafficked through Nuevo Laredo during the 2000s.[20][21]
The conflict
With an agreement between the Sinaloa cartel and the Mexican authorities, the Mexican Army managed to arrest Osiel Cárdenas Guillén. Following the 2003 arrest of Gulf Cartel leader Osiel Cárdenas, it is believed the Sinaloa Cartel moved 200 men into the region to battle the Gulf Cartel for control.[18][19] The Nuevo Laredo region is an important drug trafficking corridor as 40% of all Mexican exports, a total of 9,000 trucks, pass through the region into the United States.[19] Following the 2004 assassination of journalist Roberto Javier Mora García from El Mañana newspaper, much of the local media was silenced over the fighting.[19] The cartels intimidated the media and sometimes use it to send messages to the general population. After the latter ordered El Chapo's brother killed in 2004.[22] The killing set off a chain of reprisals and led to an escalation of the violence between the two groups in 2005.
On October 10, 2004, in Nuevo Laredo, Tamaulipas state, the bodies of five executed men were found in a house with two messages: "Send us more idiots to kill" and "This is for you, Chapo Guzmán".[23] In 2005, the homicide rate tripled in Nuevo Laredo, and President Fox sent thousands of police forces to calm the situation, causing a violent reaction of the cartels. On June 8 2005, Alejandro Domínguez Coello, chief of police of Nuevo Laredo (first ever day as a chief), was killed during a battle between the two cartels;[24] in the aftermath the military and police took over the city, resulting in 50 deaths.[25] Los Zetas turf conflict also instilled terror against journalists and civilians of Nuevo Laredo. This set a new precedent, which cartels later mimicked.[26] It is estimated that in the first eight months of 2005, about 110 people died in Nuevo Laredo, Tamaulipas, as a result of the fighting between the Gulf and Sinaloa cartels.[27] The same year, there was another surge in violence in the state of Michoacán as La Familia Michoacana drug cartel established itself after splintering from its former allies, the Gulf Cartel and Los Zetas.[28] After the split of Los Zetas from the Gulf Cartel, the Gulf Cartel and Sinaloa Cartel made an agreement to stop the conflict and fight Los Zetas.[29]
Following the contested 2006 presidential election, Felipe Calderón initiated Operation Michoacán, a militarized campaign against drug cartels as an effort to consolidate political authority, strengthen the legitimacy of his administration, and rally public support.[30][31] In 2007 the Joint Operation Nuevo León-Tamaulipas started to stop Gulf Cartel’s influence and violence in the State. [32] On April 12 2007, for the first time in the state of Nuevo León, army soldiers confronted cartel gunmen while searching houses in the municipality of Marín. One gunman was reported killed and two were arrested.[33]
Los Zetas split from Gulf Cartel
Los Zetas became so powerful that they outnumbered and outclassed the Gulf Cartel in revenue, membership, and influence by 2010.[34] As a result of this imbalance, the Cartel tried to curtail their own enforcers' influence and ended up instigating a civil war.[35] When the hostilities began, the Cartel joined forces with its former rivals, the Sinaloa Cartel and La Familia Michoacana, aiming to take out Los Zetas.[36][37]
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Antonio Cárdenas Guillén † 
Heriberto Lazcano Lazcano 
Joaquín "El Chapo" Guzmán 