MS-13 originated with a flood of young immigrants to the United States in the ‘80s. There's evidence the gang started even earlier, in the '70s, with the punk rock and heavy metal scenes, but the gang as it's known today began with the Diaspora of refugees who fled El Salvador during a bloody civil war. Between 1980 and 1990, the population of Salvadoran immigrants in the US increased from 94,000 to 465,000. Many settled in the Los Angeles area.
A lot of these immigrants were young men who grew up around violence, so naturally gravitated towards it. When they arrived in LA, they were thrown into a system of established Latin (mostly Mexican) gangs, where they weren't welcome; they had to prove themselves as a force to be reckoned with. The word “mara” is a Central American term for gang, “salva” comes from El Salvador, and “trucha” is slang for clever. The term salvatrucha is historically used to described peasants trained as guerilla fighters.
MS-13 is not a homogenous group, but is broken into cliques united under a transnational identity. The gang is believed to have started in Pico-Union, adjacent to downtown Los Angeles, though many cliques existed in the greater LA area in the early '90s. Among the first was La Fulton, which began in 1991 in the San Fernando Valley. Its leader’s nickname was Satan. La Fulton was fighting for control of the area with more than 75 other gangs. Eventually, they made an alliance with the Mexican mafia and their notoriety grew throughout the city.