On 23 August 1914, "Cossack" was involved in a collision with her sister Tribal-class destroyer, "Ghurka". In October 1914, "Cossack" was one of a number of warships of the Dover Patrol that were deployed to help support Belgian ground forces during the Battle of the Yser, with all available ships being used to carry out shore bombardment operations. At one stage, on 20 October 1914, after the destroyer "Amazon" was damaged by German shellfire, Rear Admiral Horace Hood transferred his flag to "Cossack".