[1] They were the brainchild of Admiral Sir John ("Jacky") Fisher, the man who had sponsored the construction of the world's first "all-big-gun" warship, HMS Dreadnought.

After Admiral Fisher was appointed First Sea Lord on 20 October 1904, he pushed through the Board of Admiralty in early December 1904 a decision to arm the next armoured cruiser with 12-inch (305 mm) guns and that it would have a speed no less than 25.5 knots (47.2 km/h; 29.3 mph).

This, in turn, was approved by the Board on March the 16th with only minor changes, such as the reduction in the anti-torpedo boat armament from twenty to eighteen 12-pdr guns.

[5] Early in the design process the "Committee on Designs" had thought to power these ships with the traditional reciprocating vertical triple-expansion steam engines, but were persuaded to adopt Parsons steam turbines as they required fewer boilers for the same amount of power, were easier to protect from damage as they were more compact than reciprocating engines and could be kept below the waterline.

This greatly increased the effectiveness of the rudder and substantially decreased the turning circle of the Invincibles in comparison to earlier ships of their size.

They fired 850-pound (390 kg) projectiles at a muzzle velocity of 2,725 ft/s (831 m/s); at 13.5°, this provided a maximum range of 16,450 m (17,990 yd) with armour-piercing (AP) 2 crh shells.

Firing trials against Hero in 1907 revealed this system's vulnerability to gunfire as the spotting top was hit twice and a large splinter severed the voice pipe and all wiring running along the mast.

Invincible was the first battlecruiser to receive this system during her refit from April to August 1914, but its installation was interrupted by the outbreak of the war and it was not fully working until after the Battle of the Falkland Islands in November.

Indomitable, accompanied by Indefatigable, under the command of Admiral Sir Archibald Berkeley Milne encountered the 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.

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; 14 mph), where Goeben had been spotted eight hours earlier.

In a twenty-minute bombardment, a single shell struck the magazine of the fort at Sedd el Bahr at the tip of the Gallipoli peninsula, displacing (but not destroying) 10 guns and killing 86 Turkish soldiers.

[34] Invincible's first action was as part of the battlecruiser force under the command of Admiral Beatty during the Battle of Heligoland Bight operation on 28 August 1914.

They turned south at full speed at 11:35[Note 1] when the British light forces failed to disengage on schedule and the rising tide meant that German capital ships would be able to clear the bar at the mouth of the Jade estuary.

She fired 18 rounds, all misses,[36] before Beatty's main body encountered the crippled Cöln shortly after turning north and she was sunk by two salvos from Lion.

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 SMS Leipzig was lagging behind the other ships and Inflexible opened fire on her when the range dropped to 17,500 yards (16.0 km) at 12:55.

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.

By 10:48 Blücher had been heavily damaged by fire from all the other battlecruisers and her speed had dropped to 17 knots (31 km/h; 20 mph) and her steering gear had been jammed; Beatty ordered Indomitable to attack her.

[32] After the end of the battle Indomitable was ordered to tow Lion back to port as one of her engines had been knocked out, the other was failing and she'd been holed a number of times beneath the waterline.

Invincible sailed to England and joined the 3rd Battlecruiser Squadron while Inflexible arrived at the Dardanelles on 24 January 1915 where she replaced Indefatigable as the flagship of the Mediterranean Fleet.

She had to be beached at the island of Bozcaada (Tenedos) to prevent her sinking, as she'd 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.

[34] Towards the end of the year, the British battlecruiser force was organised into three squadrons, with the 3rd BCS consisting of the three Invincible-class ships under the command of Rear Admiral Horace Hood in Invincible.

Invincible's speed was reduced to 12 knots (22 km/h; 14 mph) through flooding and she was forced to haul out of line and proceed independently to Rosyth for repairs which lasted until 22 May.

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.

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 Invincible's midships 'Q' turret, flash detonated the magazines below, and the ship blew up and broke in half, killing all but 6 of her crew of 1,032 officers and men, including Hood.

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

[54] 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.

[55] Further planned expansion included Inflexible and Indomitable, but when the secret negotiations to acquire them were leaked to the press, a major uproar erupted in Chile.