of evidence. When the Secretary lodged his formal claim of privilege, he invoked a privilege against revealing military secrets – one which is well established in the law of evidence. When a claim of privilege against revealing military secrets is invoked, the courts must decide whether the occasion for invoking the privilege is appropriate, and yet do so without jeopardizing the security which the privilege was meant to protect. When the formal claim of privilege was filed by the Secretary, under the circumstances indicating a reasonable possibility that military secrets were involved, there was a sufficient showing of privilege to cut off further demand for the documents on the showing of necessity for its compulsion that had been made. In this case, the showing of necessity was greatly minimized by plaintiffs' rejection of the Judge Advocate General's offer to make the surviving crew member available for examination. The doctrine in the criminal field that the Government can invoke its evidence privileges only at the price of letting the defendant go free has no application in a civil forum, where the Government is not the moving party, but is a defendant only on terms to which it has consented. In a suit under the Tort Claims Act, the District Court entered a judgment against the Government. The Court of Appeals affirmed. The Supreme Court reversed and remanded. Facts and background A military aircraft on a flight to test secret electronic equipment crashed, and certain civilian observers aboard were killed. Their widows sued the United States under the Federal Tort Claims Act and moved under Rule 34 of the Federal Rules of Civil Procedure for production of the Air Force's accident investigation report and statements made by surviving crew members during the investigation. The Secretary of the Air Force filed a formal claim of privilege, stating that the matters were privileged against disclosure under the Air Force regulations issued under R. S. section 161, and that the aircraft and its personnel were "engaged in a highly secret mission." The Judge Advocate General filed an affidavit stating that the material could not be furnished "without seriously hampering national security," but he offered to produce the surviving crew members for examination by the plaintiffs and to permit them to testify as to all matters except those of a "classified nature." In the absence of the documents which the Air Force failed to produce, the