President of United States is privy to lots of top secret (in a colloquial, not technical sense) information.
What are the rules and mechanisms in play to prevent a former POTUS from:
Divulging that information to other governments
Using that information themselves
Actually becoming the president or prime minister of another country (i.e., are there any international customs to prevent this; or does the United States have any authority to say "no, you can't become president over there because it is a conflict of our interests"?).