Elliot Ackerman's new book about the US exit from Afghanistan is called The Fifth Act.
But what comes next for America in the region?
On GZERO World, the former US marine and CIA officer tells Ian Bremmer that the next big challenge will be how free nations of the world respond to the rise of authoritarian regimes.
Watch the GZERO World episode: The fallout from US Afghanistan withdrawal: a Marine's perspective