Both ships played a small role in World War I as they unsuccessfully hunted for the German East Asia Squadron in late 1914.

It included the 1904 War Naval Supplementary Program which authorized construction of two battleships and four armored cruisers, among other ships.

[1] Based on the experience at the Battle of the Yellow Sea in August 1904 where the Russians opened fire at ranges well beyond what had been anticipated before the war, the IJN decided to arm the ships with 12-inch (305 mm) 45-calibre (45 feet long barrel) guns, which outranged the 12" 40-calibre guns used by the Japanese battleships in the war.

[2][3] The increase in armament was also justified by a change in the IJN's doctrine for these ships in which they were now intended to participate in the line of battle and overpower the enemy's screening armored cruisers.

[4] The Tsukubas were "briefly the world's most powerful cruisers in service until the completion of the first true battlecruisers, the British Invincible class".

[6] The Tsukuba-class design was very similar to that of the British Cressy-class armored cruiser, albeit some 1,750 long tons (1,778 t) larger.

[9] The Tsukuba-class ships had two 4-cylinder vertical triple-expansion steam engines, each driving a single propeller shaft.

[12] They fired 850-pound (386 kg) projectiles at a muzzle velocity of 2,800 ft/s (850 m/s);[13] this provided a maximum range of 24,000 yd (22,000 m) with armor-piercing (AP) shells.

[14] Their secondary armament consisted of a dozen Elswick Ordnance Company "Pattern GG" 45-caliber[15] 6-inch (152 mm) guns mounted in armored casemates on the middle and main decks.

[17] Close-range defense against torpedo boats consisted of twelve quick-firing (QF) 4.7-inch 41st Year Type guns.

[23] The Yokosuka Naval Arsenal had the most experience in building warships, but the IJN feared a bombardment by the Russian 2nd and 3rd Pacific Squadrons then en route from the Baltic Sea and decided to build the Tsukubas at the less-exposed Kure Naval Arsenal, even though Kure's experience was with ship repair and conversions.

This meant that skilled workers had to be brought from Yokosuka to train the workforce at Kure in construction techniques.

Ikoma took an additional year to finish as the end of the war shortly after she was laid down reduced the pressure to complete her as fast as possible.

Construction of both ships was somewhat delayed by difficulties in procuring enough steel plates and rivets; quantities of both had to be imported from the United States.

By 1917, both ships were assigned to the 2nd Division[34] and Tsukuba was sunk by a magazine explosion on 14 January with the loss of 305 crewmen; her wreck was later salvaged and scrapped.

She was re-rated back to first-class cruiser in 1921 and disarmed in 1922 to fulfill the requirements of the Washington Naval Treaty.