Operation Top Hat was a "local field exercise"[1] conducted by the United States Army Chemical Corps in 1953.
In June 1953 the United States Army formally adopted guidelines regarding the use of human subjects in chemical, biological , or radiological testing and research.
[1] The guidelines were adopted per an Army Chief of Staff memo (MM 385) and closely mirrored the Nuremberg Code.
[1] These guidelines also required that all research projects involving human subjects receive approval from the Secretary of the Army.
[1][2] In a 1975 Pentagon Inspector General's report, the military maintained Top Hat was not subject to the guidelines requiring approval because it was a "line of duty" exercise in the Chemical Corps.