The earliest recording of this date is found in the Samguk Sagi(History of the Three Kingdoms of Korea), the compilation in the 12th century. Current archeological evidence indicates that while a polity may have been established even earlier than this in the Gyeongju region, it is too early to call it a kingdom. The author of the Samguk Sagi, Kim Bu-sik, probably attempted to legitimize Shilla rule by giving it historical seniority over its rival kingdoms Baekje and Goguryeo.
During the Proto-Three Kingdoms period, the city-states of central and southern Korea were grouped into three confederacies called Samhan. Shilla began as Saro-guk, a statelet within the 12-member confederacy called Jinhan. Saro-guk consisted of six villages and six clans.
According to Samguk Sagi, Shilla was founded by King Park Hyeokgeose in 57 BCE, around present-day Gyeongju. Hyeokgeose is said to have been hatched from an egg laid from a white horse, and when he turned 13, six clans submitted to him as king and established Saro (or Seona). He is also the progenitor of the Park(박) clan, now one of the most common family names in Korea.
In the early years, leadership rotated among the three strongest clans, Park, Seok, and Kim. By the 2nd century, Shilla existed as a distinct state in the southeastern area of the Korean peninsula. It expanded its influence over neighboring Jinhan chiefdoms, but through the 3rd century, it was probably no more than the strongest city-state in a loose federation. To the west, Baekje had centralized into a kingdom by about 250, by overtaking the Mahan confederacy. To the southwest, Byeonhan was being replaced by the Gaya confederacy. In northern Korea, Goguryeo, a kingdom by about 50 CE, destroyed the last Chinese commandery Lelang in 313, and had grown into a threatening regional power.
King Naemul (356–402) of the Kim clan established a hereditary monarchy, eliminating the rotating power-sharing scheme, and the leader's now truly royal title became Maripgan. In 377, it sent emissaries to China, in 400 established relations with Goguryeo. Facing pressure from Baekje in the west and Japan in the south, in the later part of the 4th century, Shilla allied with Goguryeo. However, when Goguryeo began to expand its territory southward, moving its capital to Pyongyang in 427, Nulji was forced to ally with Baekje. By the time of King Beopheung (514–540), Shilla was a full-fledged kingdom, with Buddhism as state religion, and its own era name systems. Shilla absorbed the Gaya confederacy during the Gaya–Shilla Wars, annexing Geumgwan Gaya in 532 and conquering Daegaya in 562, thereby expanding its borders to the Nakdong River basin. King Jinheung (540–576) established a strong military force. Shilla helped Baekje drive Goguryeo out of the Han River (Seoul) territory, and then wrested control of the entire strategic region from Baekje in 553, breaching the 120-year Baekje-Shilla alliance. also King Jinheung was establishment Hwarang. The early period ended with the demise of the “hallowed bone” (seonggol) rank with the death of Queen Jindeok.
In the 7th century Shilla allied itself with the Chinese Tang dynasty. In 660, under King Muyeol (654-661), Shilla subjugated Baekje. In 668, under King Munmu (King Muyeol's successor) and the General Kim Yu-shin, Shilla conquered Goguryeo to its north. Shilla then fought for nearly a decade to expel Chinese forces on the peninsula intent on creating Tang colonies there to finally establish a unified kingdom as far north as modern Pyongyang. The northern region of the defunct Goguryeo state later reemerged as Balhae.
Shilla's middle period is characterized by the rising power of the monarchy at the expense of the jingol nobility. This was made possible by the new wealth and prestige garnered as a result of Shilla's unification of the peninsula, as well as the monarchy's successful suppression of several armed aristocratic revolts following early upon unification, which afforded the king the opportunity of purging the most powerful families and rivals to central authority. Further, for a brief period of about a century from the late 7th to late 8th centuries the monarchy made an attempt to divest aristocratic officialdom of their landed base by instituting a system of salary payments, or office land (jikjeon 직전, 職田), in lieu of the former system whereby aristocratic officials were given grants of land to exploit as salary (the so–called tax villages, or nogeup 녹읍, 祿邑). By the late 8th century, however, these royal initiatives had failed to check the power of the entrenched aristocracy. The mid to late 8th century saw renewed revolts led by branches of the Kim clan which effectively limited royal authority. Most prominent of these was a revolt led by Kim Daegong that persisted for three years. One key evidence of the erosion of kingly authority was the rescinding of the office land system and the reinstitution of the former tax village system as salary land for aristocratic officialdom in 757. The middle period of Shilla came to an end with the assassination of King Hyegong in 780, terminating the kingly line of succession of King Muyeol, the architect of Shilla's unification of the peninsula. Hyegong‘s demise was a bloody one, the culmination of an extended civil war involving most of the kingdom‘s high–ranking noble families. With Hyegong‘s death, during the remaining years of Shilla the king was reduced to little more than a figurehead as powerful aristocratic families became increasingly independent of central control. Thereafter the Shilla kingship was fixed in the house of King Wonseong (785–798), though the office itself was continually contested among various branches of the Kim lineage. Nevertheless, the middle period of Shilla witnessed the state at its zenith, the brief consolidation of royal power, and the attempt to institute a Chinese style bureaucratic system. Decline and Fall The final century and a half of the Shilla state was one of nearly constant upheaval and civil war as the king was reduced to little more than figurehead and powerful aristocratic families rose to dominance in the countryside. The tail end of this period, called the Later Three Kingdoms, saw the emergence of the kingdoms of Later Baekje and Later Goguryeo and Shilla's submission to the Goryeo dynasty.