As to the distinction between ICC and customary complicity standards, point gratefully taken. The U.S. Code standard is generally construed to be purpose (see, e.g., United States v. Brown, 151 F.3d 476, 486 (6th Cir.1998)), but since a prosecution may — at least in principle — take place in Spain or somewhere else, your point is quite relevant.

As to your larger point: The absence of complicity prosecutions for good-faith, non-frivolous legal opinions, combined with well-known professional responsibility norms that shield these and that tend to structure the legal community’s understanding of the lawyer’s role (at least in the U.S.), provides some indication that legal systems perceive advice of this character as lacking either the requisite actus reus or the requisite mens rea for complicity. One possible explanation is that lawyers, even in the private sector, are typically thought to be “put on the spot,” without having chosen the questions put to them or the organization’s specific goals. (This might mean that the purpose/knowledge distinction is determinative, and that, disturbingly, guilt or innocence may depend on whether the lawyers are tried under U.S. law by a U.S. court or under CIL by a foreign court.) At any rate, I am put off by the thought of convicting the OLC lawyers under a standard of lawyer liability that has so little pedigree, both because it seems an unfair surprise and because the convicts could claim victimhood, thereby distracting from more profound issues. However, if your proposal were for a prospective clarification of attorneys’ penal liabilities across the board, I might endorse it.

Two comments. First, as a matter of ICL, it is incorrect to assert that “[a]n accomplice ordinarily must have the purpose of promoting or facilitating the offense.” That is true under the Rome Statute, but it is clearly not the customary position, which is that the accused must only have knowledge that his assistance will facilitate an international crime. The knowledge standard has been consistently employed by the post-WW II tribunals, the ICTY/ICTR, and numerous domestic courts applying ICL.

Second, I think you need to address Jens’ basic point in the essay — that nothing in international law (or domestic law, for that matter) indicates that normal principles of accomplice liability do not apply to lawyers. You clearly assume that the contrary is true, such as when you write that “where the ‘facilitation’ consists solely in communicating a good-faith, non-frivolous opinion that the conduct is lawful, the result amounts to strict penal liability for mere legal error.” Indeed it does — just as the age-old maxim “ignorantia legis nihil excusat” holds non-lawyers strictly liable for “mere legal error,” as well. The (necessary) fiction that everyone knows the law is much less fictional for lawyers, so why would we give them a “true” mistake of law defence when we don’t give one to anyone else?