 The oil made the islands enormously important to the Japanese (see below), so they sought to secure the supply for themselves.They sent four fleet carriers and a light carrier along with the four fast battleships of the , 13 heavy cruisers and many light cruisers and destroyers to support their amphibious assaults in addition to conducting raids on cities, naval units and shipping in both that area and around the Indian Ocean.Access to oil was one of the linchpins of the Japanese war effort, as Japan has no native source of oil; it could not even produce enough to meet even 10% of its needs, even with the extraction of oil shale in Manchuria using the Fushun process.