at the beginning of world war i in august, "active" and her flotilla were assigned to the grand fleet. on 1 september, a submarine was spotted inside scapa flow and the 2nd df was detailed to hunt down the imaginary submarine while the rest of the grand fleet put to sea. when not escorting the capital ships of the grand fleet, the flotilla spent much time on anti-submarine patrol off the entrances to scapa flow. in mid-october, multiple reports of submarines in the minch caused admiral sir john jellicoe, commander of the grand fleet, to send "active" and some of her destroyers there to hunt them down, but nothing was found. after their search was concluded, they joined a large part of the grand fleet at lough swilly, ireland, on 22 october. sometime between october 1915 and january 1916, the cruiser was relieved of her assignment with the 2nd df and she was on detached service with the grand fleet in january.