The ‘situation’ between Iraq and the UK had long been under the scrutiny of the International Criminal Court (ICC). As early as 2004, the Office of the Prosecutor (OTP) received communications from individuals and NGO’s – as Amnesty International and Human Rights Watch –, regarding the launching of military operations and the resulting human loss in Iraq, and therefore a preliminary examination into Iraq/UK situation was opened. On 09 February 2006, the former ICC Prosecutor Luis Moreno- Ocampo, after having analysed over 240 communications, information and divergent sources documentation, in accordance with the proprio motu powers under Article 15 of the Rome Statute, announced the closure of the preliminary examination into the Iraq/UK situation. As Moreno-Ocampo explained in his letter to these groups, the reasoning for the closure was that the required “gravity” threshold of Article 17 of the Rome Statute was not met, since isolated war crimes with a small-scale number of victims are not sufficiently grave to be admissible under the ICC statutory law. On 10 January 2014, the OTP received a new communication from ECCHR and PIL, entitled “Responsibility of Officials of the United Kingdom for War Crimes Involving Systematic Detainee Abuse in Iraq from 2003-2008.” In response, on 13 May 2014, Moreno-Ocampo’s successor, Fatou Bensouda, re-opened the preliminary examination. Since then, the OTP has received a total of 236 communications or additional submissions pursuant to Article 15 of the Rome Statute (par. 14 Final Report, hereinafter also “report”). The hopes of the advocacy groups that this would lead to a different outcome did not, however, materialize. On 09 December 2020 Bensouda decided to close the preliminary examination for the second time the reasons for which are comprehensively explained in her report. This post will consider three procedural aspects of ICC practice highlighted by the Iraq/UK inquiry and how this inquiry highlights the critical importance of the principle of complementarity. Specifically, it will include: 1) the alleged war crimes and the issues of jurisdictions; 2) issues of admissibility; 3) the complementarity in the light of the UK’s military and national investigations.