64. […] [C]ustomary international rules do protect the national security of States by prohibiting every State from interfering with or intruding into the domestic jurisdiction, including national security matters, of other States. These rules are reflected in Article 2, paragraph 7, of the United Nations Charter with regard to the relations between MemberStates of the United Nations and the Organization. However, Article 2, paragraph 7, of the Charter provides for a significant exception to the impenetrability of the realm of domestic jurisdiction in respect of Chapter VII enforcement measures[1]. As the Statute of the International Tribunal has been adopted pursuant to this very Chapter, it can pierce that realm.

Furthermore, although it is true that the rules of customary international law may become relevant where the Statute is silent on a particular point, such as the “act of State” doctrine, there is no need to resort to these rules where the Statute contains an explicit provision on the matter, as is the case with Article 29. Considering the very nature of the innovative and sweeping obligation laid down in Article 29, and its undeniable effects on State sovereignty and national security, it cannot be argued that the omission of exceptions in its formulation was the result of an oversight. Had the “founding fathers” intended to place restrictions upon this obligation they would have done so, as they did in the case of Article 21, paragraph 4 (g). Article 29 therefore clearly and deliberately derogates from the customary international rules upon which Croatia relies. […]

65. […] The International Tribunal was established for the prosecution of persons responsible for war crimes, crimes against humanity and genocide; these are crimes related to armed conflict and military operations. It is, therefore, evident that military documents or other evidentiary material connected with military operations may be of crucial importance, either for the Prosecutor or the defence, to prove or disprove the alleged culpability of an indictee, particularly when command responsibility is involved (in this case military documents may be needed to establish or disprove the chain of command, the degree of control over the troops exercised by a military commander, the extent to which he was cognisant of the actions undertaken by his subordinates, etc.). To admit that a State holding such documents may unilaterally assert national security claims and refuse to surrender those documents could lead to the stultification of international criminal proceedings: those documents might prove crucial for deciding whether the accused is innocent or guilty. The very raison d’être of the International Tribunal would then be undermined.

[…]

67. Having asserted the basic principle that States may not withhold documents because of national security concerns, the Appeals Chamber wishes, however, to add that the International Tribunal should not be unmindful of legitimate State concerns related to national security, the more so because […] the International Tribunal has already taken security concerns into account in its Rules 66 (C) and 77 (B).

The best way of reconciling, in keeping with the general guidelines provided by Rule 89 (B) and (D), the authority of the International Tribunal to order and obtain from States all documents directly relevant to trial proceedings, and the legitimate demands of States concerning national security, has been rightly indicated by the Trial Chamber in the Subpoena Decision, where it suggested that in camera, ex parte proceedings might be held so as to scrutinise the validity of States’ national security claims. The Appeals Chamber, while adopting the same approach, will now suggest practical methods and procedures that may differ from those recommended by the Trial Chamber.

See also paras 66, 68-69.