“To Be of Service” shows the enduring costs of war through the lives of Vietnam, Iraq, and Afghanistan war veterans. The documentary — directed by Oscar nominee and native St. Louisan Josh Aronson (“Sound and Fury,” “Orchestra of Exiles”) — profiles veterans suffering from post-traumatic stress disorder (PTSD) who are paired with a service dog to help them regain their lives. Returning vets often experience serious depression and a wrenching disconnect from the world they once inhabited: Family members, old friends, and jobs seem foreign, and the veterans struggle to function and to resume a normal civilian life. Service dogs offer hope in helping them return to an independent, feeling life. Vets who have struggled for years with substance abuse and suicide attempts find that bonding with a service dog helps them find a way back to independence — a goal they were unable to achieve with talk therapy and prescription medications. Almost miraculously, through the unconditional acceptance and love of their service dogs, the veterans are able to re-establish their lives and family connections in a way that proves both uplifting and deeply moving.