In April 1915 the British Admiralty requisitioned "Alcantara" and her "A-series" sisters , and "Andes" to be armed merchant cruisers. She was armed with eight 6-inch guns, two six-pounder anti-aircraft guns, and depth charges. On 17 April at Liverpool she was commissioned into the Royal Navy's 10th Cruiser Squadron as HMS "Alcantara", with the pennant number M 94. "Arlanza" and "Andes" 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.