Operation Paperclip after WWII:
https://en.wikipedia.org/wiki/Operation_Paperclip

MKUltra,
https://en.wikipedia.org/wiki/Project_MKUltra

Agent Orange cover-up

Air America in Cambodia
https://en.wikipedia.org/wiki/Air_America_(airline)

Ronald Raygun's October Surprise

Phoenix Program
https://en.wikipedia.org/wiki/Phoenix_Program