Yahagi, the second Japanese warship of that name, was the third of the four Agano-class vessels (the other three being Agano, Noshiro and Sakawa). Construction of the Agano-class cruisers was approved by the Japanese parliament (Diet) in March 1939 under the Fourth Naval Armaments Enhancement Program (Dai-Yon-Ji Kaigun Gunbi Jujitsu Keikaku), also known as Four-in-Circle Program (Maru Yon Keikaku), or simply Maru 4. Under the terms of the program, the four light cruisers (kei junyokan), also referred to as type B cruisers (otsu-gata junyokan, or simply otsu jun) and officially classed as second-class cruisers (ni-to junyokan), were to fulfill the role of destroyer squadron flagships. At that time destroyer squadrons (DesRon), called literarily torpedo squadrons (suirai sentai), consisted of four four-ship destroyer divisions (DesDiv, or kuchiku-tai).