From the Document: "The security of the United States is imperiled by a drastic surge of illegal activity on the southern border. Large quantities of fentanyl, other opioids, and other dangerous and illicit drugs are flowing across our southern border and into our country at unprecedented levels, destroying the lives of our families and loved ones. Mara Salvatrucha (MS-13) and other deadly transnational gangs are systematically exploiting our unsecured southern border to enter our country and develop operational capacity in American communities throughout the country. The anticipated rapid rise in illegal crossings as we head into the spring and summer months threatens to overwhelm our Nation's law enforcement capacities."
Compilation of Presidential Documents: http://www.gpo.gov/fdsys/browse/collection.action?collectionCode=CPD
Daily Comp. Pres. Docs., 2018 DCPD No. 00218