A few years ago, my mom told me that when my dad returned from serving in Vietnam, it took a while for him to stop throwing himself under the bed when he heard a helicopter flying above. Whenever I think about that, I wonder what experiences like the ones that produce such a reaction do to a person’s mental health.

Of course, when my dad returned from war in the late 1960s, mental health was rarely discussed in the armed forces, and there was little, if any, acknowledgement that mental...