Goryeo was a Korean state founded in 918, during a time of national division called the Later Three Kingdoms period, that unified and ruled the Korean Peninsula until the establishment of Joseon in 1392. Goryeo achieved what has been called a "true national unification" by Korean historians as it not only unified the Later Three Kingdoms but also incorporated much of the ruling class of the northern kingdom of Balhae, who had origins in Goguryeo of the earlier Three Kingdoms of Korea. According to Korean historians, it was during the Goryeo period that the individual identities of Goguryeo, Baekje and Silla were successfully merged into a single entity that became the basis of the modern-day Korean identity. The name "Korea" is derived from the name of Goryeo, also romanized as Koryŏ, which was first used in the early 5th century by Goguryeo; Goryeo was a successor state to Later Goguryeo and Goguryeo.