Details are sketchy concerning the beginning stages of the Silla Empire, but we know by the sixth century it was a highly complex, lineage-based society where pedigree decided everything from the types of clothes one would wear to the jobs they'd have. While this system helped the empire initially gain land, it would eventually lead to its fall.
The Silla Empire began in 57 B.C. and covered what is now North and South Korea. Kin Park Hyeokgeose was the first to reign in the region. Under his rule, Silla continually expanded the empire, conquering a number of kingdoms on the Korean Peninsula. Eventually, a monarchy was formed. The Chinese Tang Dynasty and the Silla Empire were at war in the seventh century over the northern kingdom of Goryeo, but the Silla were able to fend them off [source: Connor].
A century of civil war among high-ranking families as well as the conquered kingdoms doomed the empire. Eventually, in 935 A.D. it abdicated power and became part of the new country of Goryeo, the same kingdom it was at war with in the seventh century. Historians don't know the exact circumstances that led to the demise of the Silla Empire, but it's generally held that neighboring nations were unhappy with the kingdom's continuing expansion across the Korean Peninsula. Theories suggest a smaller, ruling class may have fought back to gain sovereignty.