Broad and deep historical causes of gang violence in Mexico and El Salvador

If you ask most people to name the historical causes of these breakdowns of social order, I think they would probably say that in Mexico, the cause of the problem is gangs supplying the US narcotics markets, while in El Salvador it's an outgrowth unhealed wounds from decades of left-right political polarization and civil war.

Is this accurate, or are there deeper causes?

Not a specialist, but insofar as I'm aware this is inaccurate in both cases.

El Salvador / MS-13

The gang basically began in Los Angeles, and then spread to El Salvador - and Mexico - when gang members got deported from the US.

The Mara Salvatrucha gang originated in Los Angeles, set up in the 1980s by Salvadoran immigrants in the city's Pico-Union neighborhood who immigrated to the United States after the Central American civil wars of the 1980s.

Originally, the gang's main purpose was to protect Salvadoran immigrants from other, more established gangs of Los Angeles, who were predominantly composed of Mexicans and African-Americans.

Many Mara Salvatrucha gang members from the Los Angeles area have been deported after being arrested. For example, Jose Abrego, a high-ranking member, was deported four times. As a result of these deportations, members of MS-13 have recruited more members in their home countries. The Los Angeles Times contends that deportation policies have contributed to the size and influence of the gang both in the United States and in Central America. According to the 2009 National Gang Threat Assessment, "The gang is estimated to have 30,000 to 50,000 members and associate members worldwide, 8,000 to 10,000 of whom reside in the United States.

Mexican Drug cartels

In Mexico, the history is longer and the gangs/cartels are indeed related to drugs. But it's at least in part tied to the DEA becoming more efficient at blocking Colombian cartels from smuggling cocaine through Florida.

Given its geographic location, Mexico has long been used as a staging and transshipment point for narcotics and contraband between Latin America and U.S. markets. Mexican bootleggers supplied alcohol to the United States gangsters throughout the duration of the Prohibition in the United States, and the onset of the illegal drug trade with the U.S. began when the prohibition came to an end in 1933. Towards the end of the 1960s, Mexican narcotic smugglers started to smuggle drugs on a major scale.

During the 1970s and early 1980s, Colombia's Pablo Escobar was the main exporter of cocaine and dealt with organized criminal networks all over the world. When enforcement efforts intensified in South Florida and the Caribbean, the Colombian organizations formed partnerships with the Mexico-based traffickers to transport cocaine through Mexico into the United States.

This was easily accomplished because Mexico had long been a major source of heroin and cannabis, and drug traffickers from Mexico had already established an infrastructure that stood ready to serve the Colombia-based traffickers. By the mid-1980s, the organizations from Mexico were well-established and reliable transporters of Colombian cocaine. At first, the Mexican gangs were paid in cash for their transportation services, but in the late 1980s, the Mexican transport organizations and the Colombian drug traffickers settled on a payment-in-product arrangement.

Transporters from Mexico usually were given 35% to 50% of each cocaine shipment. This arrangement meant that organizations from Mexico became involved in the distribution, as well as the transportation of cocaine, and became formidable traffickers in their own right. Currently, the Sinaloa Cartel and the Gulf Cartel have taken over trafficking cocaine from Colombia to the worldwide markets.

If memory serves me well there are a few pretty good documentaries floating around on the two topics. This one by Vice on MS-13, for instance, might catch your interest.