Politicians often talk loosely about terrorism as an 'existential threat', which is a vast overstatement — terrorists don't have the capability to undermine the character and essential functions of advanced nation-states. Unless those terrorists have nuclear weapons. Here's a short video narrated by former US defence secretary William Perry about a nuclear terrorist threat to the US:

Perry is these days head of the William J Perry Project, dedicated to a world in which nuclear weapons are never used again. He's the author of a new book which argues that the threat posed by nuclear weapons has been dangerously understated in recent years. Here's a review by California Governor Jerry Brown.