When a country uses a bomb, the world knows. When a country uses cyber warfare instead... well, our intelligence services say Russia did, and Vladimir Putin says it did not.
That's one of the major differences in an age that allows warfare through computers. It's no accident that security correspondent David E. Sanger calls his new book The Perfect Weapon.
It's subtitled War, Sabotage and Fear in the Cyber Age. No soldiers need apply; hackers step forward.
The author joins us to help us understand the new world of warfare.