As Douglas P. Horne details in this ebook, JFK's War with the National Security Establishment: Why Kennedy Was Assassinated, the answer is because Kennedy's ideas about foreign-policy collided with those of the U.S. national-security establishment during the height of the Cold War. In the eyes of the military and the CIA, Kennedy's policies posed a grave threat to national security.

Horne served as Chief Analyst for Military Records for the Assassination Records Review Board (ARRB). This was the federal agency that Congress established to secure the disclosure and release of JFK-related records and documents after the public outcry generated by Oliver Stone's movie JFK. Horne is the author of Inside the Assassination Records Review Board: The U.S. Government's Final Attempt to Reconcile the Conflicting Medical Evidence in the Assassination of JFK, a five-volume work that focuses primarily on the autopsy of Kennedy's body that was conducted by the U.S. military.