He worked at the RAND Corporation, inside the Pentagon directly under the Secretary of Defense, on-site in Vietnam, and then based on what he saw, decided to commit an incredibly major federal crime by going public about what was going on in the United States's decision-making process so the American people could be aware. He was charged with espionage, with the government aiming to put him in prison for 115 years, and beat the case. Much later, he wrote a book with his experiences and insights.
That's an extremely broad overview. I would highly recommend looking into more of the details, or maybe reading his book.