134. The Appeals Chamber recalls that while individual criminal responsibility generally requires the commission of a positive act, this is not an absolute requirement.[1] In particular, the Appeals Chamber has previously found that “the omission to act where there is a legal duty to act can lead to individual criminal responsibility under Article 7(1) of the Statute”.[2] Moreover, the Appeals Chamber has consistently found that, in the circumstances of a given case, the actus reus of aiding and abetting may be perpetrated through an omission.[3]

135. Accordingly, the Appeals Chamber finds that the Trial Chamber properly considered aiding and abetting by omission as a recognised mode of liability under the International Tribunal’s jurisdiction.[4]

49. At the outset, the Appeals Chamber recalls that to enter a conviction for aiding and abetting murder by omission, at a minimum, all the basic elements of aiding and abetting must be fulfilled.[1] In this regard, the Appeals Chamber in Orić recalled that “omission proper may lead to individual criminal responsibility under Article 7(1) of the Statute where there is a legal duty to act”.[2] The actus reus of aiding and abetting by omission will thus be fulfilled when it is established that the failure to discharge a legal duty assisted, encouraged or lent moral support to the perpetration of the crime and had a substantial effect on the realisation of that crime.[3] The Appeals Chamber recalls that aiding and abetting by omission implicitly requires that the accused had the ability to act, such that there were means available to the accused to fulfil his duty.[4] Meanwhile, the required mens rea for aiding and abetting by omission is that “[t]he aider and abettor must know that his omission assists in the commission of the crime of the principal perpetrator and must be aware of the essential elements of the crime which was ultimately committed by the principal”.[5] As the Appeals Chamber held in the  Simić case,

it is not necessary that the aider and abettor knows either the precise crime that was intended or the one that was, in the event, committed. If he is aware that one of a number of crimes will probably be committed, and one of those crimes is in fact committed, he has intended to facilitate the commission of that crime, and is guilty as an aider and abetter.[6]

[…].

146. As Šljivančanin argues,[7] the Appeals Chamber has never set out the elements for a conviction for omission in detail.[8] In the Orić case, the Appeals Chamber considered the Trial Chamber’s findings in order to determine whether Atif Krdžić, Naser Orić’s subordinate, had been found responsible for aiding and abetting by omission.[9] It concluded that no such finding had been entered as the issue of whether Naser Orić’s subordinate had incurred criminal responsibility had not been resolved by the Trial Chamber.[10] In this context, with regard to the mode of liability of aiding and abetting by omission, the Appeals Chamber held that:

at a minimum, the offender’s conduct would have to meet the basic elements of aiding and abetting. Thus, his omission must be directed to assist, encourage or lend moral support to the perpetration of a crime and have a substantial effect upon the perpetration of the crime (actus reus). The aider and abettor must know that his omission assists in the commission of the crime of the principal perpetrator and must be aware of the essential elements of the crime which was ultimately committed by the principal (mens rea).[11]

Accordingly, the Appeals Chamber in Orić acknowledged that the basic elements of aiding and abetting apply notwithstanding whether this form of liability is charged as “omission”. The mens rea and actus reus requirements for aiding and abetting by omission are the same as for aiding and abetting by a positive act.[12] The critical issue to be determined is whether, on the particular facts of a given case, it is established that the failure to discharge a legal duty assisted, encouraged or lent moral support to the perpetration of the crime, and had a substantial effect on it. In particular, the question as to whether an omission constitutes “substantial assistance” to the perpetration of a crime requires a fact based enquiry.[13]

93. [T]he Appeals Chamber considers that even though Šljivančanin no longer had de jure authority over the military police deployed at Ovčara, had he ordered the military police not to withdraw, these troops may well have, in effect, obeyed his order to remain there, considering he had been originally vested with the authority for the entire evacuation of the Vukovar Hospital and entrusted with responsibility for protecting the prisoners of war. In particular, Šljivančanin could have informed the military police deployed at Ovčara that Mrkšić’s order was in breach of the overriding obligation under the laws and customs of war to protect the prisoners of war, and thus constituted an illegal order.

94. Indeed, issuing an order contrary to Mrkšić’s to the military police of the 80 mtbr was a course of action that would have required Šljivančanin to go beyond the scope of his de jure authority, which had been effectively removed by virtue of Mrkšić withdrawal order.[1] Nonetheless, the illegality of Mrkšić’s order required [ljivančanin to do so. To further support this conclusion, the Appeals Chamber recalls the analysis in the Čelebići Trial Judgement which implies that in the context of preventing the commission of a war crime, an officer may be expected to act beyond the strict confines of his de jure authority:

Likewise, the finding in the High Command case that a commander may be held criminally liable for failing to prevent the execution of an illegal order issued by his superiors, which has been passed down to his subordinates independent of him, indicates that legal authority to direct the actions of subordinates is not seen as an absolute requirement for the imposition of command responsibility. Similarly, the finding in the Toyoda case, whereby the tribunal rejected the alleged importance of what it called the "theoretical" division between operational and administrative authority, may be seen as supporting the view that commanders are under an obligation to take action to prevent the commission of war crimes by troops under their control despite a lack of formal authority to do so. An officer with only operational and not administrative authority does not have formal authority to take administrative action to uphold discipline, yet in the view of the tribunal in the Toyoda case; "[t]he responsibility for discipline in the situation facing the battle commander cannot, in the view of practical military men, be placed in any hands other than his own.”[2]

Although the Trial Chamber in Čelebići discussed this in the context of superior responsibility, the Appeals Chamber considers that the principle that an officer may be required, within the limits of his capacity to act, to go beyond his de jure authority to counteract an illegal order is equally applicable to the present case.

See also footnote 331:

It is a principle of international humanitarian law that subordinates are bound not to obey manifestly illegal orders or orders that they knew were illegal. See Hostage Case (United States v. Wilhelm List et al., Trials of War Criminals, Vol. XI, p. 1236): “[T]he general rule is that members of the armed forces are bound to obey only the lawful orders of their commanding officers and they cannot escape criminal liability by obeying a command which violates international law and outrages fundamental concepts of justice”. See also Erdemović 1996 Sentencing Judgement, para. 18 (“Although the accused did not challenge the manifestly illegal order he was allegedly given, the Trial Chamber would point out that according to the case-law referred to, in such an instance, the duty was to disobey rather than to obey.”), fn. 12 (“Trial of Rear-Admiral Nisuke Masuda and four others of the Imperial Japanese Navy, Jaluit Atoll Case, U.S. Military Commission, U.S. Naval Air Base, Kwajalein Island, Kwajalein Atoll, Marshall Islands, 7-13 December 1945, Case No. 6, L.R.T.W.C., Vol. I, pp. 74-76, pp. 79-80. See also Trial of Wilhelm List and Others, U.S. Military Tribunal, Nuremberg, 8 July 1947-19 February 1948, L.R.T.W.C., Case No. 47, Vol. VIII, pp. 50-52 […].); Mrđa Sentencing Judgement, para. 67 (“As to the related issue of superior orders, Article 7(4) of the Statute states that ‛[t]he fact that an accused person acted pursuant to an order of a government or of a superior […] may be considered in mitigation of punishment if the Tribunal determines that justice so requires.’ […] [T]he orders were so manifestly unlawful that Darko Mrđa must have been well aware that they violated the most elementary laws of war and the basic dictates of humanity. The fact that he obeyed such orders, as opposed to acting on his own initiative, does not merit mitigation of punishment.”).

151. The Appeals Chamber recalls that it has previously recognised that the breach of a duty to act imposed by the laws and customs of war gives rise to individual criminal responsibility.[1] The Appeals Chamber further recalls that Šljivančanin’s duty to protect the prisoners of war was imposed by the laws and customs of war.[2] Thus, the Appeals Chamber considers that Šljivančanin’s breach of such duty gives rise to his individual criminal responsibility. Therefore, it is not necessary for the Appeals Chamber to further address whether the duty to act, which forms part of the basis of aiding and abetting by omission, must stem from a rule of criminal law.

1677. The Appeals Chamber recalls that a person may be held criminally responsible for aiding and abetting by omission where he or she fails to discharge a legal duty and by this failure assists, encourages or lends moral support to the perpetration of a crime and has a substantial effect on the commission of that crime.[1] Additionally, aiding and abetting by omission necessarily requires that the accused has “the ability to act, or in other words, that there were means available to the accused to fulfil this duty”.[2]

1678. The Appeals Chamber considers that while Lazarević’s failure to take investigative and punitive measures against the commission of forcible displacement may have had an effect on the ability of the military prosecutor to pursue perpetrators of such crimes, this in itself is not conclusive for the purposes of establishing aiding and abetting liability. Rather as recalled above, in order to fulfil the actus reus of aiding and abetting, it must be demonstrated that any such omission substantially contributed to the continued commission of forcible displacement.[3] […]

43. The Prosecution submits that the Trial Chamber found Atif Krdžić responsible for aiding and abetting by omission.[1] The Appeals Chamber recalls that omission proper may lead to individual criminal responsibility under Article 7(1) of the Statute where there is a legal duty to act.[2] The Appeals Chamber has never set out the requirements for a conviction for omission in detail.[3] However, at a minimum, the offender’s conduct would have to meet the basic elements of aiding and abetting. Thus, his omission must be directed to assist, encourage or lend moral support to the perpetration of a crime and have a substantial effect upon the perpetration of the crime (actus reus).[4] The aider and abettor must know that his omission assists in the commission of the crime of the principal perpetrator[5] and must be aware of the essential elements of the crime which was ultimately committed by the principal (mens rea).[6]

41. The Appeals Chamber considers that the Trial Chamber did not hold Atif Krdžić criminally responsible for commission by omission. At a minimum, the actus reus of commission by omission requires an elevated degree of “concrete influence”.[1] Such was not the case here, where the Trial Chamber merely found that Atif Krdžić’s absence from the detention facilities “coincide[d] with more killings and more maltreatment”.[2] Furthermore, the Trial Chamber clearly distinguished Atif Krdžić from the principal perpetrators who physically committed the crimes.[3]

109. The Appeals Chamber considers that Stanišić’s argument that the Trial Chamber failed to apply the correct legal standard to his “purported omissions”[1] is based on the premise that each failure to act assessed in the context of joint criminal enterprise liability must, per se, meet the legal conditions set out in the Tribunal’s case law in relation to commission by omission. In this respect, the Appeals Chamber recalls that although participation in a joint criminal enterprise – which is based on an accused’s significant contribution to the common criminal purpose – is a form of “commission” under Article 7(1) of the Statute, this is a mode of liability distinct from commission by omission and is characterised by different objective and subjective elements.[2]

110. In this respect, the Trial Chamber properly held that for an accused to be found criminally liable on the basis of joint criminal enterprise liability, it is sufficient that he acted in furtherance of the common purpose of a joint criminal enterprise in the sense that he significantly contributed to the commission of the crimes involved in the common purpose.[3] Beyond that, the law does not foresee specific types of conduct which per se could not be considered a contribution to a joint criminal enterprise.[4] Within these legal confines, the question of whether a failure to act could be taken into account to establish that the accused significantly contributed to a joint criminal enterprise is a question of fact to be determined on a case‑by‑case basis.[5] Furthermore, the Appeals Chamber recalls that the relevant failures to act or acts carried out in furtherance of a joint criminal enterprise need not involve carrying out any part of the actus reus of a crime forming part of the common purpose, or indeed any crime at all.[6] That is, one’s contribution to a joint criminal enterprise need not be in and of itself criminal, as long as the accused performs acts (or fails to perform acts) that in some way contribute significantly to the furtherance of the common purpose.[7] In light of the above, contrary to Stanišić’s assertion, when establishing an accused’s participation in a joint criminal enterprise through his failure to act, the existence of a legal duty to act deriving from a rule of criminal law is not required.[8] The nature of the accused’s duty and the extent of his ability to act are simply questions of evidence and not determinative of joint criminal enterprise liability.[9]

111. In the present case, as part of its factual determination of Stanišić’s contribution to the JCE, the Trial Chamber considered, together with his other actions,[10] his failure to discipline the RS MUP personnel who had committed crimes and to protect the civilian population,[11] despite his duties to do so, together with his ability, as the highest authority, to investigate and punish those who had committed crimes.[12] The Appeals Chamber observes that in the jurisprudence of the Tribunal, a failure to intervene to prevent recurrence of crimes or to halt abuses has been taken into account in assessing an accused’s contribution to a joint criminal enterprise and his intent where the accused had some power and influence or authority over the perpetrators sufficient to prevent or halt the abuses but failed to exercise such power.[13] Therefore, Stanišić has not shown that the Trial Chamber applied an erroneous legal standard when it considered instances of his failures to act in assessing whether he contributed to the JCE.

[…]

733. Turning to Župljanin’s arguments in relation to his capacity or ability to act, based on the same reasoning, the Appeals Chamber finds no merit in Župljanin’s attempt to conflate the Appeals Chamber’s statement that the objective element of commission by omission requires, at a minimum, an “elevated degree of ‘concrete influence’”,[14] with the significant contribution requirement of joint criminal enterprise liability.[15] For the same reason, the Appeals Chamber dismisses Župljanin’s argument that the Trial Chamber failed to make findings or particularised findings as to whether “he had such a high degree of control over his subordinates that his ‘omission’ could count as part of the actus reus of commission”.[16]

See also paras 731-732, 734.