Man's best friend is being recruited to heal PTSD.

After terrifying tours of duty in Kuwait, Iraq and Somalia, a retired Army Sergeant is recovering from Post Traumatic Stress Disorder with the help of a four-legged therapist, reports CBS News correspondent Richard Schlesinger.

For two solid weeks, retired Sgt. Jim Mason and Yankee have been in intensive training.

Yankee is already a fully qualified service dog. It's Mason who is in training. He's learning to use Yankee to cope with what doctors have diagnosed as severe PTSD.

"I know I can say 'hello' to anyone with that dog and they'll smile and say 'hello' back," says Mason.

And that alone is a huge step for Mason, who still feels trapped in the real life nightmares he lived in both Gulf wars and in Somalia - especially Somalia - where his unit was ambushed while trying to provide humanitarian aid.

"I had to kill some people," Mason says as he chokes up.

Fifteen years later, he still can barely talk about it.

"You know, I mean they shot at us," he said. "We were ambushed. And if we didn't fight back they were gonna kill us. But in my heart, I didn't go there for that."

When he retired after 20 years in the military, he started noticing changes in himself. He couldn't sleep, he snapped at people, and he was afraid he'd become violent.

Now, he's got Yankee - a brand new solution for a problem as old as war. He can sense when Mason gets tense and help avoid panic attacks by staying close - a four-legged security blanket. He's also trained to keep an eye out behind Mason, and barks when he sees something or someone that Mason may consider a threat.

Yankee's used to watching and being watched. He was trained by inmates in a New York penitentiary, as part of a program called Puppies Behind Bars. And what Yankee learned in prison helps him free Mason from lingering terror.

The cliché is that a dog is man's best friend. The reality is, Yankee's a lot more to Jim Mason. He's indispensable.