In 1938, Ramirez de Arellano was assigned as Division Officer of the , a Porpoise-class submarine. The "Pickerel" was training near the Philippines when on December 8, 1941 the islands were attacked by Japan. The "Pickerel" was ordered to patrol the coast of the islands and on her second war patrol she sank the "Kanko Maru", a Japanese vessel, in the Gulf of Davao off Mindanao. He participated in five war patrols with the "Pickerel" and led the effort to rescue five Navy pilots and one enlisted gunner off Wake Island. He also contributed to the sinking of two Japanese freighters and damaging a third. For his actions, he was awarded a Silver Star Medal and a Legion of Merit Medal.