Todd Bensmamn published a backgrounder monograph, What Terrorist Migration Over European Borders Can Teach About American Border Security in November 2019. The key findings are relevant not only to the United States, but for all countries in the West, struggling to survive the black magic of migration. The key points are summarised:
https://cis.org/Report/Terrorist-Migration-Over-European-Borders
https://www.youtube.com/watch?v=BmdQEDJfAWo&feature=emb_title
“At least 104 Islamist extremists entered the European Union’s (EU) external borders using long-haul irregular migration methods between 2014-2018, establishing proof-of-concept for a previously theoretical terrorism travel tactic over borders. • Of the 104 migrant terrorists identified for the 2014-2018 period, 28 successfully completed attacks that claimed the lives of 170 victims and wounded 878. An additional 37 were arrested or killed plotting attacks, and 39 others were arrested for illegal involvement with foreign terrorist organizations. • A majority of the 104 terrorists applied for international protections such as asylum and were able to remain in European nations for an average of 11 months before attacks or arrests for plots, demonstrating that asylum processes accommodated plot incubation. • While total numbers of migrant-terrorists were a fraction of overall irregular migration, their activities demonstrate that small numbers present outsized threats to the United States should only a few strike here. In Europe, a relative few upended longstanding political power balances, prompted a major military campaign, forced massive public expenditures on new security policies, prompted the UK vote to exit the European Union, and disrupted the free movement of people within the Schengen zone. • American security and counter-terrorism intelligence measures likely have prevented some migrant-terrorists from conducting attacks and may partly explain the absence of attacks to date from over the U.S. southern border. However, an elevated threat of terrorist infiltration via the border persists due to neglect and exploitable flaws in the American measures.”