1317 (Bunpō gannen文保元年): The new era name was created to mark an event or series of events. The previous era ended and the new one commenced in Shōwa 6.
Events of the Bunpō era
During this era, Negotiations between the Bakufu and the two lines resulted in an agreement to alternate the throne between the two lines every 10 years (the Bunpō Agreement). This agreement did not last very long, being broken by Emperor Go-Daigo.
1318 (Bunpō 2, 2nd month): In the 11th year of Hanazono-tennō 's reign (花園天皇11年), the emperor abdicated; and the succession (senso) was received by his cousin, the second son of former-Emperor Go-Uda. Shortly thereafter, Emperor Go-Daigo is said to have acceded to the throne (sokui).
1319 (Bunpō 3, 4th month): Emperor Go-Daigo caused the nengō to be changed to Gen'ō to mark the beginning of his reign.