the international laws of war, may be applied to cover any hostile act committed by a detainee because of the US government’s position that detainees do not qualify as combatants in accordance with international humanitarian law. Reliance on a domestic common law of war undermines the very concept of international humanitarian law and the principles enshrined in the Geneva Conventions. Convicting individuals for offences that are not defined in accordance with existing crimes at the time of the conduct or that extend liability beyond the specified conduct violates the principle of legality, which is protected by both international humanitarian and human rights laws.726 318. As the MCAs have failed to restrict their jurisdiction to offences recognized under international humanitarian law, some detainees have been tried and convicted of such offences. In al Bahlul v. United States of America, an en banc decision by the United States Court of Appeals for the District of Columbia Circuit unanimously determined that it was a plain error727 to convict al Bahlul of providing material support for terrorism and of solicitation. The Court found that military commissions could not try these offences because, at the time of al Bahlul’s conduct, they were neither violation of the laws of war nor established offences under domestic law as the relevant conduct occurred prior to the enactment of the MCA 2006.728 As a result, convicting him of these offences was a plain violation of the Ex Post Facto Clause.729 Furthermore, in June 2015 a panel of the same court vacated al Bahlul’s remaining conviction for conspiracy after finding that the conviction violated the separation of powers enshrined in the US Constitution. Stating that inchoate conspiracy is not a crime under international humanitarian law, the Court concluded that Congress exceeded its authority by subjecting domestic offenses such as of Columbia Circuit, al Bahlul v. United States of America, “Brief of the National Institute of Military Justice as Amicus Curiae in Support of the Petitioner”, 18 August 2014, <http://www.lawfareblog.com/wp-content/uploads/2014/08/al-Bahlul-PanelRemand-Amicus.pdf>; Wells Bennett, “Defense: OLC Memo Supports Invalidation of Omar Khadr’s Conviction”, Lawfare website, 2 July 2014, <http://www.lawfareblog.com/2014/07/defense-olc-memo-supports-invalidation-of-omar-khadrsconviction/>; ODIHR interview with Cheryl Bormann, Air Force Captain Michael Schwartz, Navy Reserves Lieutenant Commander James Hatcher and Tim Jon Semmerling, 28 January 2014: it was explained that the domestic law of war concept will not stand before federal courts. In relation to the applicability of international law, defence counsel stressed that if the crimes their clients are accused of are war crimes, then international law must apply. 726 See, for instance, ICCPR, Art. 15; UN HRC, General Comment No. 29, op. cit., note 111, para. 7; UN HRC, David Michael Nicholas v. Australia, op. cit., note 662, para. 7.5; Third Geneva Convention, Art. 99, Fourth Geneva Convention, Arts. 65, 67; API, Art. 75(4)(c); “Customary International Humanitarian Law, Rule 101”, ICRC website, op. cit., note 664. 727 The majority opinion in al Bahlul v. United States of America (en banc) employs the “plain error” standard of review. The “plain error standard of review” stems from the rule that generally bars the petitioner from raising an argument that was not preserved in the lower court trial, unless a plain error occurred or exceptional circumstances exist. To preserve an error, a timely objection must be made during the lower court proceedings. However, in rare cases, an appellate court can take notice and correct a plain error. To constitute a plain error, and for the appellate court to vacate the lower court’s holding, four elements must be present. A plain error occurs when there is (1) an error; (2) that is plain; (3) that affects substantial rights; and (4) “the error seriously affects the fairness, integrity or public reputation of judicial proceedings.” United States Court of Appeals for the District of Columbia Circuit, al Bahlul v. United States of America (en banc), 14 July 2014, op. cit., note 680. 728 This case referred to both international humanitarian law and the concept of US common law of war. ODIHR does not support the existence of the US common law of war. 729 As a result of this decision, the convictions for providing material support for terrorism and solicitation were vacated in July 2014. The Court also determined that the conviction for conspiracy was not a plain error. The case was remanded for further proceedings on remaining issues. In June 2015, the conspiracy conviction of al Bahlul was vacated by a panel of the Court of Appeals for the District of Columbia Circuit. Following the June 2015 ruling, the United States petitioned for a rehearing en banc of the decision. United States Court of Appeals for the District of Columbia Circuit, al Bahlul v. United States of America (en banc), 14 July 2014, op. cit., note 680; United States Court of Appeals for the District of Columbia Circuit, al Bahlul v. United States of America, 12 June 2015, op. cit., note 632; United States Court of Appeals for the District of Columbia Circuit, al Bahlul v. United States of America, “Petition of the United States for Rehearing en Banc”, 27 July 2015, op. cit., note 632. 139