Illegal Immigration
This is any foreigner who enters the U.S. without an entry or immigrant visa, especially a person who crosses the border by avoiding inspection or who overstays the period of time allowed as a visitor, tourist, or businessperson. The real Threat is those that want in to cause destruction. They must be stopped at any US border and prohibited from entering.
A strategy would be to provide a high level psychological test while hooked up to a device that would monitor emotions, heart rate and fluctuations in brain waves. If they were vetted properly we could let any immigrant in on a trial basis.