Silla was one of the three ancient kingdoms of Korea which battled with its rivals Paekche and Koguryo for hundreds of years before finally emerging victorious and ruling the entire Korean peninsula at the end of the 1st millenium AD.
According to tradition, Silla was founded by King Park Hyokkose (the progenitor of today's Park surname in Korea). Archaeology tells us that by at least by the 2nd century AD there was a loose confederacy of tribes in southeastern Korea in the area which would later become Silla. The Park family dominated the confederacy for several centuries, but was eventually overthrown by the Seok family, who themselves were finally overthrown by the Kim family.
By the 6th century, Silla had become a full-fledged kingdom, with a hereditary aristocracy, Chinese-style calendar and reign names, Buddhism as the state religion, law codes based on Chinese models, and a hierarchical system of ranks and titles.
By this time Silla was also in an almost constant state of war with the two other powerful kingdoms on the pennisula, Paekche and Koguryo. Over the course of the 6th century, Silla conquered the neighboring confederacy of Kaya, weakening Paekche who had relied upon Kaya as a trusted ally. Finally, in the 7th century, Silla concluded a strategic alliance with Tang Dynasty China, and with the help of Tang troops, was able to conquer Paekche in 660 and Koguryo in 668. Silla then spent more than a decade expelling the Tang army from Korea, as the Tang had only helped Silla because they secretly hoped to colonize the peninsula.
Silla now ruled the entire Korean peninsula for several hundred years, although it abandoned the northern-most regions of the old Koguryo kingdom, which became the kingdom of Parhae. For more than a century after the unification, Silla was an efficiently-run, highly bureaucratized state, divided into nine provinces and numerous administrative subunits.
But without the constant threat of nearby enemies to keep them on their toes, the Silla kings gradually became lazy and soft, and by the end of the 9th century the kings had been reduced to impotent figureheads as powerful provincial aristocrats carved out their own fiefs in the countryside and peasant rebellions created quasi-autonomous regions which refused to pay taxes. Finally, in 935, the last remnants of the Silla kingdom were conquered by Wang Kon who reunited the Korean peninsula and founded the Koryo kingdom.