In May 1942 the Japanese began transferring POWs by sea. Similar to treatment on the Bataan Death March, prisoners were often crammed into cargo holds with little air, food or water for journeys that would last weeks. Many died due to asphyxia, starvation or dysentery. Some POWs became delirious and unresponsive in their environment of heat, humidity, and lack of oxygen, food, and water. These unmarked prisoner transports were targeted as enemy ships by Allied submarines and aircraft.
More than 20,000 Allied POWs died at sea when the transport ships carrying them were attacked by Allied submarines and aircraft. Although Allied headquarters often knew of the presence of POWs through radio interception and code breaking, the ships were sunk because interdiction of critical strategic materials was more important than the lives of prisoners-of-war, and because Allied leaders feared that a pattern of sparing POW ships might lead the Japanese to use prisoners as human shields on valuable targets.