By the time Kennedy and Johnson held the presidency in the 1960s, the definition of US national security had been stretched and expanded in previously unimaginable ways. It was not unusual for Americans to perceive their security frontiers as global – indeed, it was considered natural. But it hadn’t always been thus. Read by Leighton Pugh. Image: Poster showing the American flag waving among clouds.  Credit: World History Archive / Alamy Stock Photo