>> worley: yes, sir. >> pelley: it's a familiar story for jon nachison, who, as a clinical psychologistith vets for almost 30 years. it's a new generation of homeless veterans. >> nachison: it is. this group is becoming homeless quicker than the vietnam veteran. vietnam vets came back, it took about eight to ten years before we started really seeing them on the street homeless. this group is coming back, and within a year, they're ending up on the street. and my best hunch is that, for many of them, it's these redeployments again and again. >> pelley: over the last ten years, almost 900,000 troops have been redeployed, sent back to combat at least once. getting redeployed two, three, four, even five times, why does that make a difference? >> nachison: when you go back and you are re-traumatized, it also brings up all the old stuff. go back again, and it layers over the top of that. and so now, we're getting to the point where it's going to be difficult for the person to function. >> pelley: more than two million troops have already served in iraq and afghanistan. the v.a. believes there