War, war crimes, and hell
Dorrigo's hard life takes him to Japan where he fights in WWII and is taken as a prisoner of war (POW). Since he is a surgeon and doctor, he becomes very powerful in the camp, since combat knowledge is not more helpful than medical knowledge in a war camp. Dorrigo's life is one of constant carnage and trauma. One day, Dorrigo finds a man being beaten, and he attempts to free the man, but the guards offer him a trade for the right to kill the prisoner: If Dorrigo does not stop them, they will give him medicine to save other dying prisoners.
All the horrible deaths and brutal violence in the novel are indications of real life. These events are factual and historically accurate, even though Dorrigo is a made up character. Therefore the purpose of the graphic, gory elements in the narrative are to illustrate that war is hell, and war brings out the worst in people, often driving them to the most hateful, horrifying behavior imaginable.
True love versus settled love
The truth of romance is that "true love" is really capricious, and even those who find it never get to really keep it. But Dorrigo is left in limbo. He really loves Amy, and since they have already been intimate with one another, they already share a deep, secret union. When he comes back from war and marries Ella, he does it because he thinks Amy is dead. Then when he learns that Amy was an option the whole time (she was not actually dead at all), he can't get over the fact that he settled for Ella.
Ella spends some of the last scenes in the book trying to forgive him for settling on her, and then cheating on her instead of trying to really enjoy their marriage for whatever is was worth. The thematic question raised by this story of failed connection is whether or not one should chase fantasies about "True love" or whether any marriage would work, given that both parties truly invest themselves in it.
Post-Trauma
None of his affairs or failures to connect with his wife and family ought to be held against Dorrigo, because Dorrigo was made to witness horrific scenes in a brutal POW camp. The untreated Post-Traumatic Stress from such an event leaves Dorrigo constantly perplexed, agitated, and unable to feel truly safe. He has been forced to see too much, and his body simply cannot forget. (Now they have EMDR treatment for these cases, but back then, the medical community didn't have much to offer by way of therapy). When Dorrigo dies, the author finishes the novel by offering a quick epilogue where he explains what happened to the other characters. All the people who went to war came back with serious mental health issues, without exception.
