Donald Trump tweeted at 5:39 a.m. on April 18 that the spread of Mara Salvatrucha (MS-13) resulted from weak immigration enforcement under Obama. The gang is actually a homegrown product whose power and extent were increased by the policy of deporting aliens with criminal convictions under Bill Clinton—the same policy that Trump boasts about continuing now.
Here's what we say in The Politics of Immigration: Questions and Answers, second edition, Chapter 8, “Are Immigrants a Threat?”:
…Deporting someone for a violent crime may actually make us less safe....[I]f the offender is truly a danger to others, deportation makes no sense. It simply takes offenders out of the parole system, and their criminal record may not follow them across the border. Whether they remain in their country of birth, go elsewhere, or return to the United States without permission, no one is monitoring them to ensure they don’t put someone else at risk….
Thousands of Central Americans were deported as “criminal aliens” after IIRIRA and AEDPA were enacted. Among them were youths who belonged to Mara Salvatrucha (known as MS-13), a Los Angeles-based street gang. Crime experts say the deportations made it possible for the U.S. gang to take root in Central America, especially in El Salvador, fueling criminal violence there. As a result of its connections in both Central America and the United States, MS-13 grew into a real force in the smuggling and distribution of drugs. In October 2012 the U.S. Treasury declared MS-13 a significant “transnational criminal organization” because of its “serious transnational criminal activities, including drug trafficking, kidnapping, human smuggling, sex trafficking, murder, assassinations, racketeering, blackmail, extortion, and immigration offenses.” MS-13 members now reportedly see deportations from the United States as a way to get “free rides” to other countries in order to expand the gang’s operations.