Inflexible bombarded Turkish forts in the Dardanelles in 1915, but was damaged by return fire and struck a mine while maneuvering.

Transferred to the Grand Fleet afterwards, she damaged the German battlecruiser Lützow during the Battle of Jutland in 1916 and watched Invincible explode.

[4] Inflexible had two paired sets of Parsons direct-drive turbines, each of which was housed in a separate engine-room and drove an outboard and inboard shaft.

[7] At full fuel capacity, she could steam for 3,090 nautical miles (5,720 km; 3,560 mi) at a speed of 10 knots (19 km/h; 12 mph).

[10] Her anti-aircraft armament consisted of a single QF 3-inch (76 mm) 20 cwt AA gun on a high-angle MKII mount at the aft end of the superstructure that was carried from July 1915.

[11] After the Battle of Jutland revealed her vulnerability to plunging shellfire, additional armour was added in the area of the magazines and to the turret roofs.

[2] She was the temporary flagship of Admiral of the Fleet Sir Edward Hobart Seymour while in New York for the Hudson–Fulton Celebration in September 1909.

Accompanied by Indefatigable, under the command of Admiral Sir Archibald Berkeley Milne she encountered the German battlecruiser Goeben and the light cruiser Breslau on the morning of 4 August 1914 headed east after a cursory bombardment of the French Algerian port of Philippeville, but Britain and Germany were not yet at war so Milne turned to shadow the Germans as they headed back to Messina to recoal.

Therefore, Milne stationed Inflexible and Indefatigable at the northern exit of the Strait of Messina, still expecting the Germans to break out to the west where they could attack French troop transports, the light cruiser Gloucester at the southern exit and sent Indomitable to recoal at Bizerte where she was better positioned to react to a German sortie into the Western Mediterranean.

Milne, still expecting Rear Admiral Wilhelm Souchon to turn west, kept the battlecruisers at Malta until shortly after midnight on 8 August when he set sail for Cape Matapan at a leisurely 12 knots (22 km/h), where Goeben had been spotted eight hours earlier.

Finally on 9 August, Milne was given clear orders to "chase Goeben which had passed Cape Matapan on the 7th steering north-east".

Milne still did not believe that Souchon was heading for the Dardanelles, and so he resolved to guard the exit from the Aegean, unaware that the Goeben did not intend to come out.

They departed on 11 November and rendezvoused with several other cruisers under Rear Admiral Stoddard at Abrolhos Rocks, off the coast of Brazil on the 26th.

They were spotted at 07:30, although the pre-dreadnought Canopus, grounded in Stanley Harbor to defend the town and its wireless station, did not receive the signal until 07:45.

The Germans were not expecting any resistance and the first salvo from Canopus's guns at 09:20 caused them to sheer off from their planned bombardment of the wireless station and fall back on Spee's main body.

The light cruiser Leipzig was lagging behind the other ships and Inflexible opened fire on her when the range dropped to 17,500 yards (16,000 m) at 12:55.

Eight minutes later, Spee turned again to the east to give battle; this time, his strategy was to close the range on the British ships so he could bring his 15 cm (5.9 in) secondary armament to bear.

Spee turned to the northwest, as if to attempt to cross the British T, but actually to bring Scharnhorst's undamaged starboard guns to bear as most of those on his port side were out of action.

Gneisenau had been slowed by earlier damage and was battered for another hour and a half by Inflexible and Invincible at ranges down to 4,000 yards (3,700 m).

A 240 mm (9.4 in) shell hit the foremast at the same level as the flying bridge and set fire to the navigator's sea cabin.

[25] Later, as she was turning in Eren Keui Bay, she was seriously damaged by a mine – probably about 100 kg (220 lb) in size – that blew a large hole in her starboard bow and flooded the forward torpedo flat, drowning 39 men.

She had to be beached at the island of Bozcaada (Tenedos) to prevent her sinking, as she had taken in some 1,600 long tons (1,600 t) of water, but she was temporarily repaired with a cofferdam over the 30-by-26-foot (9.1 m × 7.9 m) hole.

Hood interpreted this as an attempt to escape through the Skagerrak and ordered an increase in speed to 22 knots (41 km/h; 25 mph) at 15:11 and steered East-Southeast to cut off the fleeing ships.

The 2nd Scouting Group was escorted by the light cruiser Regensburg and 31 destroyers of the 2nd and 9th Flotillas and the 12th Half-Flotilla which attacked the 3rd BCS in succession.

As Invincible turned north, her helm jammed and she had to come to a stop to fix the problem, but this was quickly done and the squadron reformed heading west.

A 305 mm (12-inch) shell from the third salvo struck the roof of Invincible's midships 'Q' turret, flash detonated the magazines below, and the ship blew up and broke in two, killing all but six of her crew of 1,032 officers and men, including Rear-Admiral Hood.

Seydlitz was hit five times before the German battlecruisers were rescued by the appearance of the pre-dreadnought battleships of Rear Admiral Mauve and the British shifted fire to the new threat.

[31] The loss of three battlecruisers at Jutland (the others were Queen Mary and Indefatigable) led to the force being reorganised into two squadrons, with Inflexible and Indomitable in the 2nd BCS.

However, after Jutland there was little significant naval activity for the Invincibles, other than routine patrolling, thanks to the Kaiser's order that his ships should not be allowed to go to sea unless assured of victory.

It was a series of collisions which led to the loss of two K boats, serious damage to three others, including K22, and a cruiser, and the deaths of 104 submariners, with no enemy involved.