The U.S. homeland faces a multilayered threat from terrorist organizations. Homegrown jihadists account for most of the terrorist activity in the United States since 9/11. Efforts by jihadist terrorist organizations to inspire terrorist attacks in the United States have thus far yielded meager results. No American jihadist group has emerged to sustain a terrorist campaign, and there is no evidence of an active jihadist underground to support a continuing terrorist holy war. The United States has invested significant resources in preventing terrorist attacks, and authorities have been able to uncover and thwart most of the terrorist plots. This Perspective identifies 86 plots to carry out terrorist attacks and 22 actual attacks since 9/11 involving 178 planners and perpetrators. Eighty-seven percent of those planners and perpetrators had long residencies in the United States. Only four of them had come to the United States illegally, all as minors. Nationality is a poor predictor of later terrorist activity, and vetting people coming to the United States, no matter how rigorous, cannot identify those who radicalize here. Determining whether a young teenager might, more than 12 years later, turn out to be a jihadist terrorist would require the bureaucratic equivalent of divine foresight.

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