Nothing grants an ex-president, or even a serving president immunity from prosecution. The obstacles that would need to be overcome would be public will, public and foreign perception, foreign relations, fiscal justification and judicial need. Mostly it's a matter of public will though. The US is obviously proud of its power and would hate to see that waned. There would be a lot of resistance to seeing even the least of it's presidents prosecuted for an action taken in the name of national security.
Nothing grants an ex-president, or even a serving president immunity from prosecution. The obstacles that would need to be overcome would be public will, public and foreign perception, foreign relations, fiscal justification and judicial need. Mostly it's a matter of public will though. The US is obviously proud of its power and would hate to see that waned. There would be a lot of resistance to seeing even the least of it's presidents prosecuted for an action taken in the name of national security.