"You can't get over combat experience," Millard told me in an interview in the Washington DC home he shares with three other veterans.

"You don't get over it, you don't move on, you learn to live with it."

The base where he carried out military intelligence work was shelled relentlessly, targetted by enemy insurgents' mortar fire.

"You can try and be as tough as you want, [but] the first time you get mortared in a combat zone, it's terrifying," Millard said.

He has been diagnosed with post-traumatic stress disorder (PTSD) and has been unable to hold down a job for more than a year.

"You go from being terrified of these mortars to not caring less. But then when you come back here that desensitisation wears off. I have panic attacks, and sometimes I can't breathe … It's tough to have a job when so many things send you into a panic."

Invisible wounds