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.”).