Using theoretical insights from International Law and International Relations, this paper examines US policies in the early post 9/11 period with respect to the treatment of detainees captured in Iraq, Afghanistan and elsewhere. The paper concludes that while these policies seem to have been driven primarily by political considerations of national security, international legal rules pertaining to the treatment of detainees were also marginalised in key policy debates partly because of the insufficient or uneven internalisation of these rules in domestic political and bureaucratic circles. Focusing on the theoretical work of Harold Koh, this paper suggests that to better understand how international law works in practice it would be useful to expand Koh's analysis of the processes of internalisation to allow for disparities in the evenness with which international legal rules might be internalised within domestic bureaucracies, and for the variable relationship between political and bureaucratic internalisation.