adelaer was a personal friend of the new dutch supreme commander lieutenant-admiral michiel de ruyter who also had been knighted in denmark, for his victory over sweden in 1659. a large part of their correspondence in dutch has survived. in 1675 denmark joined the dutch in the franco-dutch war; sweden then declared war. adelaer commanded a single minor action against the swedish fleet — the only time he would actually fight in danish service — but during an epidemic that swept scandinavia that fall, adelaer was afflicted and died after many weeks of suffering on 5 november 1675 at copenhagen. he was replaced as supreme commander on 8 may 1676 by a dutch admiral, cornelis tromp.