Bob everything you're asking about is answered in the link that was posted (and which I'm repasting) above. You.re really making a mistake by not at least looking at it.
Article I was Obstruction of Justice. Section 3 reads:
- approving, condoning, acquiescing in, and counselling witnesses with respect to the giving of false or misleading statements to lawfully authorized investigative officers and employees of the United States and false or misleading testimony in duly instituted judicial and congressional proceedings;
Section 5:
- endeavouring to misuse the Central Intelligence Agency, an agency of the United States;
The "national security" issue Nixon talks about on tape is the Cuban Bay of Pigs invasion ten years earlier. That the investigation might "open that whole thing up again." Open what up again? That didn't make too much sense and is undoubtedly the reason the FBI Director Patrick Gray didn't buy it. Why Congress accused him of trying to "misuse the CIA."
Where you seem to be getting confused is, Nixon didn't try and use the national security claim as a legal defense. He tried to use executive privilege. Essentially he argued the president is above the law. Which is what I was referring to -- if the president does it it's not illegal -- which started this whole segueway.