According to Unbroken, why were the Japanese so harsh to the POWs? What did they hope to achieve?
I would argue that the Japanese did not really “hope to achieve” anything by treating their prisoners of war (POWs) as badly as they did. They were not doing it as a way to get something. Instead, they were doing it because their culture and/or their psychology told them they should. Let us look at what Hillenbrand has to say on this matter.
One place where Hillenbrand discusses the reasons for the Japanese treatment of POWs is in Chapter 19. (I cannot give page numbers as I have this book in electronic form without page numbers.) Hillenbrand tells us that the Japanese military “applauded brutality” and that soldiers were routinely beaten. She says that the soldiers who ended up guarding POWs were typically those with the lowest status. They would “vent their frustration” at their low status and at being beaten on the POWs since the POWs could not resist. Japanese historians call this “transfer of oppression.”
Hillenbrand then goes on to argue that there were two more factors that reinforced this tendency. First, the Japanese felt that they were racially superior to their prisoners. This made them more likely to abuse those prisoners. Second, and perhaps most importantly, the idea of surrender was terribly shameful in Japanese society at that time. Hillenbrand discusses how Japanese soldiers were told in training that they should not allow themselves to be captured because that would shame them and their families. Instead, they should die fighting. (Hillenbrand does not discuss this, but there was a Japanese word “gyokusai” that meant “honorable death” or “death without surrender” and was made up of the characters for “jewel” and “smash” or “crush.” This gave the idea that soldiers who died rather than surrendering were like jewels.) Because the Japanese thought that it was shameful to surrender, they despised men who had surrendered. Since the POWs had surrendered, the Japanese felt they had lost their honor and their right to be treated with dignity.
What all this tells us is that the Japanese did not treat POWs badly because they wanted to achieve anything. They were brutal to their prisoners because that was what their society said they should do and because they were psychologically inclined to treat their prisoners horribly.