He, along with John Kerry and Donald Rumsfeld, let down the more than 100 POWs still in Southeast Asia, mostly Laos, in 1991, when the three said there was no credible evidence any any surviving POws in Southeast Asia. There existed ground and/or aerial photographic evidence for each of the more than 100 POWs.
Just like President Eisenhower let down the more than 100 Korean POWs when no effort was made to negotiate their release or rescue. We know the more than 100 here, because, after the armistice was signed, more than 100 POWs in the North escaped custody and crossed into South Korea or were otherwise rescued.