 In January 1915 , British artillery arrived in Belgrade , further bolstering its defences . On 22 April 1915 , a British picket boat that had been brought overland by rail from Salonika was used to attack the Danube Flotilla anchorage at Zemun , firing two torpedoes without success . Bulgaria joined the Central Powers in September 1915 , and the Serbian Army soon faced an overwhelming Austro @-@ Hungarian , German and Bulgarian ground invasion . On 7 October , the Austro @-@ Hungarian 3rd Army attacked Belgrade , and Körös , along with the majority of the flotilla , was heavily engaged in support of the crossings near the Belgrade Fortress and Ada Ciganlija island . During the final river crossing and support of the resulting bridgehead , the ship provided close support , during which her stack was hit and damaged . The following day , Körös assisted SMS Enns when the latter took a direct hit and began to take on water . 
