Completed in 1916, she was assigned to the Dover Patrol where her primary duties involved bombarding German targets on the coast of occupied Belgium, particularly at the ports of Zeebrugge and Ostend.

During the First World War, the Royal Navy developed several classes of ships which were designed to give close support to troops ashore through the use of naval bombardment.

The monitor's engines would not start reliably and were prone to stalling,[4] making it impossible for the ship to achieve even the minimum standard for the trial of four hours of continuous sailing at full power.

Some of the main modifications were an increase in the power supply to guarantee a speed of 12 knots (22 km/h; 14 mph) and a change to the angles and lines of the hull to improve steering.

On 24 September the ship made an attempt to bombard the port of Zeebrugge but this was soon aborted when the weather conditions prevented the accurate observation of the fall of shot and any subsequent correction of trajectory.

[22] In early 1917, Terror and the rest of the patrol made several aborted attempts to bombard the lock gates of the Bruges Canal at Zeebrugge but the operation had to be postponed each time.

The German shore battery began to return fire but ceased after four rounds due to the same poor visibility, enhanced by a British smoke screen.

[26][22] Interrupted by German air raids and the rising tide, the repair crew opted to leave the holes in the bow substantially open and focused instead on reinforcing the bulkheads.

The monitor departed Dover on the evening of 23 October but encountered rough seas off Hastings at around midnight; the bow began to take on water and the ability to steer deteriorated.

[22] On the morning of 21 March 1918 the monitor helped foil a German raid on Dunkirk by nine destroyers and ten torpedo boats, which had been intended to support the Spring Offensive.

[22] On 27 September, Terror and Erebus provided gunnery support for the Fifth Battle of Ypres; their objective being to bombard positions around Zeebrugge and Ostend, with the aim of diverting German attention from other fronts.

[18] As a result of increased tensions in the Pacific Ocean, following Japan's withdrawal from the League of Nations, Terror was overhauled and recommissioned with new armaments before being dispatched to Singapore in 1933.

[37][38] She departed Plymouth on 9 October 1933, in company with the salvaged fishing trawler Fastnet and the barrage vessel Sandgate,[18][vii] to begin the journey to the Far East.

Fastnet was damaged twice during the voyage, spending three weeks in the dockyard at Gibraltar and receiving a temporary patch at Sabang in Sumatra to allow it to limp the short distance to Singapore.

Terror also had an adventure while passing through the Suez Canal; a tow rope was cast off too soon, causing the ship to swing broadside across the channel with the bow and stern touching either bank.

[39] Terror arrived at Singapore on the evening of 14 January 1934 along with the heavy cruiser Kent, flagship of the China Station, destroyers Veteran and Wren and the aircraft carrier Eagle.

[41] In 1937 the monitor participated in gunnery trials alongside Singapore's newly completed 15 in guns, in order to develop the shore defences' capabilities with indirect fire.

[44] In May 1939 the Colonial Office suggested moving Terror to the West Indies in order to protect the oil fields of Trinidad from growing internal threats, such as an emboldened labour movement and anti-colonial sentiment in the region.

[46] At the end of the month she left the Royal Navy base at Alexandria to lend the strength of her guns to Malta, which the British felt was vulnerable to an expected Italian attack.

[49] For three months the monitor helped to defend Malta from daily air attacks until the arrival of additional anti-aircraft guns allowed a refit of her armour to begin on 4 September.

[50][19] She was stationed in the bay on 3 December when two Italian aircraft dropped torpedoes into the harbour, one of which hit and badly damaged the stern of the light cruiser Glasgow.

The barrels were inspected by Vice Admiral Sir Andrew Cunningham and the order was given for Terror to reduce the amount of cordite used when firing the main guns, in an attempt to extend the weapons' useful life.

[54] In a further attempt to conserve the monitor's main guns, her duties were changed to concentrate on providing anti-aircraft cover for the rest of the squadron and to ferry supplies from Alexandria.

[58] Along with the flotilla leader Stuart, the gunboat Gnat and the destroyers Vampire and Voyager, Terror supported the assault on Tobruk on 21 January by the 6th Australian Division with the port being secured on 22nd.

Intending to use the port as a key supply point for their armies, the allies recognised that the shortage of anti-aircraft guns, lack of radar and limited air cover from the RAF was a strategic weakness that would have to be addressed.

[62] While Peony made steady progress in clearing the harbour, work was slow due to the shallow waters reducing the effectiveness of her degaussing equipment.

[63] Despite the monitor's efforts to protect the harbour using her anti-aircraft guns, the convoy was unable to offload the majority of its cargo due to the frequent air attacks and sailed on to Tobruk on 20 February.

Haynes protested the lack of air support to Cunningham along with the dangers of remaining at anchor, stating that it was only a matter of time before Terror received a direct hit.

[65] As they were exiting the harbour the monitor triggered two magnetic mines in the previously cleared channel; although again not a direct hit, the explosions in close proximity caused further damage and flooding.

[68][ix] Following the sinking, Commander Haynes was awarded the Distinguished Service Order in August 1941 for "courage, skill and devotion to duty in operations off the Libyan Coast."