Trade secrets
How the non-disclosure agreement extended from trade secrets to secrets of life and death, of rape and pillage, is wrapped up with another strand of the story of the cold war. For it was the security state that innovated in using the NDA as a powerful tool to keep things out of court.
In 1984, for instance, the Reagan administration was forced to form a commission to investigate the murder of four nuns in El Salvador. The nuns were lefties, and their murder was planned and executed, it was suspected, by allies, to say the least, of the Americans. The study was completed, and turned into the State department, which promptly branded it top secret.
The families of the nuns protested. This protest was heard. The administration then took a compromise position. The family members could see the report, as long as they signed an NDA that pledged them to never disclose the information in the report under any circumstances as long as they shall live without the permission of the State Department.