During World War I, when the British government applied to borrow four Kongō-class battlecruisers, the Japanese government and the Ministry of Foreign Affairs were at a loss. At that time, apart from the Kongō-class battlecruisers, Japan had only the Fusō as a decent battleship. The rest were second-class main force ships such as the dreadnoughts and pre-dreadnoughts. For this reason, the Japanese government and the Ministry of Foreign Affairs believed that the Imperial Japanese Navy, being the concerned party, would not agree to the British proposal.
However, contrary to expectations, the Imperial Navy accepted the loan of the Kongō-class battlecruisers with conditions. The condition was that if the borrowed Kongōs suffered any losses, instead of providing replacement ships, monetary compensation should be given to the Navy. The money would be allocated to the construction cost for replenishing the lost ship. The officials in the Japanese government and the Ministry of Foreign Affairs, who wished to maintain a good relationship with the United Kingdom, were relieved by the Imperial Navy's response.
However, the government and the Ministry of Foreign Affairs were unaware that the Imperial Navy was already planning fleet development with 41-centimeter gun-equipped battleships through the redesign of the Fusō-class battleships. As a result, the Kongō-class battlecruisers, which were now considered outdated battleships by the naval high command due to the difficulty of refitting them with 41-centimeter guns, were not known to the government or the Ministry of Foreign Affairs.
. . .
The four Kongō-class battlecruisers dispatched to the United Kingdom faced a harsh fate. In the year of their arrival in Europe, both the Kongō and Hiei were successively sunk by the German submarine U-21. Additionally, the Haruna and Kirishima suffered significant damage in a fierce battle with German battleships in the Battle of Jutland. In the end, only the Haruna and Kirishima returned to Japan intact, and that too more than six months after the war had ended. However, it wasn't all bad. Technicians sent from Japan to the UK had the opportunity to acquire advanced British shipbuilding technology through the repair of the Haruna and Kirishima.
The suffering during World War I was not exclusive to the Kongō-class battlecruisers. The Japanese destroyer fleet, dispatched in response to the British request, engaged in deadly battles with U-boats in European seas, including the Mediterranean, and suffered significant losses. For Japanese destroyers lacking effective submarine detection devices, capturing lurking U-boats in the sea was extremely challenging. Even with sharp-eyed Japanese soldiers, visual surveillance had its limits. Nevertheless, Japanese destroyers, dedicated to protecting Allied merchant ships, fought tenaciously and received high praise from naval forces worldwide. When the Imperial Navy was offered the knowledge and technology gained by the British during World War I regarding anti-submarine warfare, Japan willingly accepted, recognizing the dedication of its destroyer fleet.
Japan suffered considerable losses in valuable cruisers and destroyers during World War I, but it also understood the importance of protecting sea lanes and the terror of submarines. At the same time, the Japanese navy was surprised and intrigued by the first aerial attack using a torpedo and its results. A seaplane, weighing less than three tons, launched from a British seaplane carrier and sank ships of several thousand tons with the torpedo it carried.
Considering the agility of aircraft capable of rapid three-dimensional maneuvers compared to surface vessels limited to moving on the water, the vulnerability of the latter became evident. If a multitude of aircraft carrying torpedoes were to swarm toward Japan's fleet without potent anti-aircraft weapons, the consequences would be disastrous. There were two countermeasures: equipping vessels with excellent fire control systems capable of intercepting and shooting down small aircraft carrying torpedoes and having aircraft capable of repelling enemy planes.
With this, the Imperial Navy realized that even technically immature submarines had proven their ability to destroy battleships. Similarly, aircraft, just starting to emerge, would rapidly evolve into large, high-speed, and highly destructive assets with enhanced engines. Although the Navy's main force, battleships, would continue to evolve in terms of size and speed, limitations were inevitable due to budget and shipyard constraints. Moreover, nations struggled with the enormous construction and maintenance costs associated with battleships. The large-scale and high-speed evolution of battleships would only exacerbate these challenges. Additionally, if battleships became more defensively robust, submarines and aircraft would likely carry torpedoes and bombs of equivalent destructive power. It was evident that submarines and aircraft had more room for development compared to battleships.
In any case, against submarines capable of three-dimensional stealth maneuvers underwater and aircraft with high-speed aerial mobility, surface vessels moving openly on the water were at a clear disadvantage. Urgent measures were necessary, but for the current Imperial Navy, it posed a formidable challenge given limitations in personnel and budget. Sacrifices in some aspect had to be made.