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Rh people, most of whom use the platforms for interactions that once took place via mail, on the phone, or in public areas. The fact that some bad actors took advantage of these platforms is insufficient to state a claim that defendants knowingly gave substantial assistance and thereby aided and abetted those wrongdoers’ acts. And that is particularly true because a contrary holding would effectively hold any sort of communication provider liable for any sort of wrongdoing merely for knowing that the wrongdoers were using its services and failing to stop them. That conclusion would run roughshod over the typical limits on tort liability and take aiding and abetting far beyond its essential culpability moorings.

In holding that plaintiffs had stated a claim, the Ninth Circuit went astray through a series of missteps that, together, obscured the essence of aiding-and-abetting liability. To correct those errors, we proceed through the Ninth Circuit’s application of Halberstam’s framework.

The Ninth Circuit framed the issue of substantial assistance as turning on defendants’ assistance to ISIS’ activities in general. See 2 F. 4th, at 909. But, as we explained above, the question is whether defendants gave substantial assistance to ISIS with respect to the Reina attack. The focus thus must remain on the Reina attack; plaintiffs’ failure to allege any definable nexus between the defendants’ assistance and that attack therefore—at minimum—drastically increases their burden to show that defendants somehow consciously and culpably assisted the attack.

Next, the Ninth Circuit misapplied the “knowing” half of “knowing and substantial assistance.” It first separated the “knowing” and “substantial” subelements; it then analyzed the “knowing” subelement as a carbon copy of the antecedent element of whether the defendants were “generally aware” of their role in ISIS’ overall scheme. Ibid.; see also