Between 17 and 20 July 1914, "Thunderer" took part in a test mobilisation and fleet review as part of the British response to the July Crisis. Arriving in Portland on 25 July, she was ordered to proceed with the rest of the Home Fleet to Scapa Flow four days later to safeguard the fleet from a possible surprise attack by the Imperial German Navy. In August 1914, following the outbreak of World War I, the Home Fleet was reorganised as the Grand Fleet, and placed under the command of Admiral Jellicoe. Repeated reports of submarines in Scapa Flow led Jellicoe to conclude that the defences there were inadequate and he ordered that the Grand Fleet be dispersed to other bases until the defences be reinforced. On 16 October the 2nd BS was sent to Loch na Keal on the western coast of Scotland. The squadron departed for gunnery practice off the northern coast of Ireland on the morning of 27 October and the dreadnought struck a mine, laid a few days earlier by the German armed merchant cruiser . Thinking that the ship had been torpedoed by a submarine, the other dreadnoughts were ordered away from the area, while smaller ships rendered assistance. On the evening of 22 November 1914, the Grand Fleet conducted a fruitless sweep in the southern half of the North Sea; "Thunderer" stood with the main body in support of Vice-Admiral David Beatty's 1st Battlecruiser Squadron. The fleet was back in port in Scapa Flow by 27 November. On 8 December, she sailed to Devonport for a brief refit to fix problems with her condensers.