Document ID: ./input/supremecourt_opinions/opinions/22pdf/21-1496_d18f.pdf
Page Number: 33.0

Cite as:  598 U. S. ____ (2023) 

27 

Opinion of the Court 

people,  most  of  whom  use  the  platforms  for  interactions
that once took place via mail, on the phone, or in public ar-
eas.  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  particu-
larly 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 culpa-
bility moorings. 

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

The Ninth Circuit framed the issue of substantial assis-
tance as turning on defendants’ assistance to ISIS’ activi-
ties 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’ fail-
ure to allege any definable nexus between the defendants’ 
assistance and that attack therefore—at minimum—dras-
tically  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 anteced-
ent  element  of  whether  the  defendants  were  “generally 
aware” of their role in ISIS’ overall scheme.  Ibid.; see also