In April 1915 the Admiralty requisitioned "Arlanza" and her "A-series" sisters , "Andes" and for conversion into armed merchant cruisers. "Arlanza" was armed with six guns, and commissioned at Birkenhead on 24 April 1915 into the 10th Cruiser Squadron. "Andes" and "Alcantara" were also commissioned into the 10th Cruiser Squadron, which joined the Northern Patrol that was part of the First World War Allied naval blockade of the Central Powers. The Squadron patrolled about of the North Sea, Norwegian Sea and Arctic Ocean, to prevent German access to or from the North Atlantic.