During the Japanese occupation of the Dutch East Indies, a recruitment for Imperial Japanese Navy (IJN) officers was held in Medan. Nasution joined the recruitment program, and underwent naval educations in Bandung and Kyoto. He was finally given the rank of lieutenant and became the steersman in the Haraokamaru ship in 1944. He participated in the Pacific Theater and fought against the US forces in the Solomon Islands campaign and in the Philippines campaign. During a naval battle in the Philippines, his ship was hit and caught fire. Nasution was injured and almost died.