The Imperial Japanese Navy responded in part by the construction of auxiliary vessels, such as fleet oilers and submarine tenders, designed so that they could be converted quickly into aircraft carriers in time of conflict.

Inadequate sectioning into waterproof compartments below her waterline, combined with the weak construction of her hull, also made the ship vulnerable in combat situations.

Immediately after the launching ceremony, Taigei was returned to the dry dock for repairs and modifications, which involved replacement of damaged sections by the traditional rivet construction method.

Seawater ingression from faulty waterproof doors shorted the electric system, disabling her steering and the waves from the typhoon cracked a number of the welds in her hull.

Shortly before the start of hostilities in the Pacific War, Taigei was ordered back to Japan for conversion into a light aircraft carrier, arriving at Kure on 4 December.

On her first mission on 11 December, under the command of Captain Yoshio Kamei, she was sent to the Japanese naval base at Truk escorted by the destroyer Tokitsukaze.

At 09:10 on 12 December, she was hit by a single torpedo on the starboard side from the American submarine USS Drum near Hachijojima, and was immediately forced to return to Yokosuka for emergency repairs, and remained out of operation until early 1943.

[2] On 19 March Ryūhō began a series of uneventful aircraft ferry missions to occupied islands in the South Pacific.

On 25 November, she departed with Hiyō and escorts on a long circular patrol and training mission, sailing to Manila, then to Singapore, then to Tarakan, then Palau, then Truk, then Saipan, and finally returning to Kure on 2 January 1944.

From there, she sailed with the Combined Fleet to participate in the Battle of the Philippine Sea as part of "Force B" (with the carriers Hiyō, Junyō, the battleship Nagato, the cruiser Mogami, and eight destroyers).

On 25 October, with the escort carrier Kaiyō, Ryūhō set sail from Sasebo Naval District on another aircraft ferry mission to Keelung, Formosa.

[2] Upon reaching Formosa and unloading her cargo, Ryūhō was among the targets of a major series of American carrier-based air raids all over the island.