Were mistakes made in the preparation of the controversial OLC memoranda? There is no question about it; they were very serious mistakes, and they have harmed this country. But I think part of the explanation is that the authors of the "torture" memoranda lacked sufficient expertise in national security law, and honestly believed that a conflict involving more than seventy-five sovereign nations was "international" in scope. They reasonably read the language in Common Article 3 limiting its application to conflicts "occurring in the territory of one of the High Contracting Parties" as excluding its applicability to a struggle taking place across much of the world. And the fact that the D.C. Circuit Court of Appeals unanimously accepted this reasoning (before being overturned by a divided Supreme Court) would seem to be prima facie evidence that the interpretation was reasonable.
Robert F. Turner, What Went Wrong? Torture and the Office of Legal Counsel in the Bush Administration, 32 Campbell L. Rev. 529 (2010).