Clint Hill, former Secret Service agent for Jacqueline Kennedy, said that as a little boy, John "loved the military."

"'We would go out to Middleburg where [the Kennedys] had leased this property, and where they eventually built a place, and he would play soldier in the woods around the house,' Hill says. 'He had his nanny, she would put on her nurse's uniform and she would be the military nurse. And he would take a stick that he would find and he used that as a rifle and kind of march around like he was a soldier counting cadence just like he heard the military do.'"

https://people.com/politics/jackie-kennedy-secret-service-agent-remembers-jfk-jr-birth/