During the First Anglo-Dutch War of 1652–1654, De Ruyter agreed to join the expanding Dutch fleet as a junior flag-officer or "commandeur", a rank broadly comparable to that of commodore, commanding a Zeelandic squadron of "director's ships", which were privately financed warships, after he had initially refused the post on the grounds that others were better qualified for it. De Ruyter proved his worth under the supreme commander lieutenant-admiral Maarten Tromp. The rank of admiral-general was reserved for the stadtholder, but at the time, no-one held that appointment. The main function of De Ruyter's squadron was to convoy outbound or returning Dutch merchant ships through the English Channel, where they were vulnerable to attacks from English ships based at Portsmouth or Plymouth.