The Yōrō Ritsuryo established the Iwase Province in 718 through the division of the Michinoku Province ([[Mutsu Province). It was composed of five districts of Shirakawa (白河), Iwase (石背), Aizu (会津), Asaka (安積) and Shinobu (信夫). It returned to Michinoku some time between 722 and 724.