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.
This happened to a friend of mine. He was undocumented but doing fine, riding a bus from one place to another, and didn’t realize that when the bus wandered a little close to the southern border, CBP could essentially pull the bus over and asked to see everyone’s papers. And that’s exactly what they did. TL;DR long detention in jail and a big fight to be able to stay in the country.
In the current system, he’d probably already be gone, and under Trump 2, someone in that situation might have something really really bad happen to them.