map of Unified Silla and Balhae (8th century)
By the mid-sixth century, the Silla Kingdom had brought under its control all of the neighboring town-states within the Gaya Confederation.
Through an alliance with the Tang Dynasty of China, Silla unified the Korean Peninsula in 668 and saw the zenith of its power and prosperity in the mid-eighth century. It attempted to establish an ideal Buddhist country. Bulguksa Temple was constructed during the Unified Silla period. However, its Buddhist social order began to deteriorate as the nobility indulged in increasing luxury.
Silla had repelled Tang attempts to subjugate Goguryeo and Baeche by 676. Then in 698, the former people of Goguryeo who resided in south-central Manchuria established the Kingdom of Balhae. Balhae included not only people of Goguryeo, but also a large Malgal population.
Balhae established a government system centered around five regional capitals, which was modeled after the Goguryeo Kingdom's administrative structure. Balhae possessed an advanced culture which was rooted in that of Goguryeo.
Balhae prosperity reached its height in the first half of the ninth century with the occupation of a vast territory reaching to the Amur River in the north and Kaiyuan in south-central Manchuria to the west. It also established diplomatic ties with Turkey and Japan. Balhae existed until 926, when it was overthrown by the Khitan. Many of the Balhae nobility, who were mostly Goguryeo descendants, moved south and joined the newly founded Goryeo Dynasty.
