The notorious gang known as MS-13 has become a focal point in President Donald Trump’s rhetoric on immigration, with the president citing the gang’s violence to call for a crackdown on borders and a ramp-up of deportations.

But how did MS-13 come to be in the first place?

In this video drawn from FRONTLINE’s reporting for the new documentary, The Gang Crackdown, learn how the United States itself played a key role — first when it backed El Salvador’s government in a brutal civil war that displaced scores of civilians, and then when President Bill Clinton fast-tracked deportations in an attempt to stem gang violence.

For more on MS-13, watch The Gang Crackdown.

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