Peter Bergen wrote for the Washington Post about Trump's travel ban and its ineffectiveness:
The same pattern holds true in the United States. According to research by New America, a Washington-based think tank, of the 13 perpetrators of lethal jihadist terrorist attacks in the States since 9/11 (which killed a total of 94 people), all were American citizens or legal permanent residents. Of the 406 cases of jihadist terrorism (nonlethal and otherwise) in this country since Sept. 11, 2001, tracked by New America, more than 80 percent involved U.S. citizens or legal permanent residents.
None of the lethal terrorists were refugees, nor were any of them from any of the six countries the Trump administration wants to suspend travel from, nor were their families from any of those countries, and only one was a relatively recent immigrant (from Pakistan, which is not on the travel ban list).
There is no single pathway that turns someone into a terrorist who kills strangers in the name of Allah, but often jihadist terrorists in the West are second-generation immigrants who do not feel quite fully British or French or American, but who also do not feel entirely at home in the cultures of their Middle Eastern or South Asian parents. This creates an identity crisis, which a small minority resolves by turning to a militant form of Islam that is usually quite distinct from the religious practices of their families. Some go on to embrace violent jihad. In this twisted strain of Islam, violent radicals are offered acceptance, respect — and even glory.