Loss
The main theme of the novel is loss, and it comes in many different forms. There is the most obvious form - the loss of friends during the war, or the loss of friendships that disintegrate because the war is taking its toll on a person's character, making them change. There is also a loss of family, when the characters working in Vietnam are so far away from theirs that they might as well be on a different planet as in a different country.
The most obvious example of a character experiencing loss is Kathy Jones; she experiences her own loss, as a young widow, and also experiences the loss of others, as an Adoption Coordinator for Vietnamese orphans. Each aspect of loss is woven into the lives of the characters, and is not necessarily confined to the loss of a loved one or other person. For example, Skip experiences the loss of trust in his uncle, and the loss of the man that he had believed him to be.
The Colonel and the Houston brothers also experience the loss of self, as the Colonel really does begin to lose his mind, and become a shadow of his former self, and both Bill and James lose their sense of identity, turning to drink and petty crime when they cannot cope with their return to normal life.
Estrangement
Somewhat similar to loss, the theme of estrangement sees both those separated by miles and oceans, and those separated by character experiencing a distancing from those they love. The Houston brothers are estranged from their families and at first they intend to keep them alive in their minds, and to connect with them on a practical level by sending home money, but as the carnage and violence around them takes its toll, they begin to become more estranged from their previous lives, spending the money, and making only rare or sporadic contact.
Civility
Although this is technically another loss, the gradual disappearance of civility from the lives of the characters in the novel is insidious but constant. Eventually each experiences a "them or me" kind of survivalist instinct as the realization dawns that their life has become a kill or be killed situation, altering the behavior and character of each of them.
