Brandon Bryant's story provides a rare glimpse into a secret world and raises questions about the nature of 21st Century warfare.

He joined the US Air Force straight out of college. He was picked to join one of the United States' most controversial and important military programmes - the deployment of armed unmanned aerial vehicles, better known as drones - to hunt down and kill some of America's most dangerous enemies.

The experience has left him haunted and angry. We find out why.