Sogaya(small state), along with Ara-Gaya was a major political force that formed the federation of Gaya states (A.D. 350 ? A.D. 562). To this day, most researchers as well as public believed that the territory of Sogaya was the area around today’s Goseong in Gyeongsangnam-do.
However, Sogaya was most likely the term that referred to many tribal states that existed in the western part of today’s Gyeongsangnam-do area, including Goseong.
The result of recent archeological researches shows that the western part of Gyeongsangnam-do area exhibits a significant level of spiritual and physical homogeneity in terms of forms of pottery, tombs and funeral rituals. However, it is not plausible to think that Goseong was a center of development of such a wide cultural band. The distribution of tombs indicates that many small states existed around several areas that include Goseong, Jinju and Sancheong. It appears that these small states occupied a common cultural band.
Based on pottery forms, tomb style and distribution of tombs, the development of Sogaya could be described in terms of three stages: foundation period (A.D 400~475), period of growth (A.D 475~525), period of decline (A.D 525~560)
Pottery culture unique to Sogaya began to form around A.D. 5th century and geographic characteristics become prominent as time passed. During the growth period, production and distribution of Sogaya’s potteries took place in many regions: the Nam-gang river area including Sancheong and Jinju; the southern sea coast area including Goseong, Tongyoung, Sacheon and Hadong; and Uiryeong, Masan and the eastern part of Jeollanamdo. However, by the middle of the 6th century, the production of Sogaya’s potteries rapidly declines in volume and is limited to Sancheong, Jinju, Goseong and Sacheon area. Then, around the fall of Daegaya in A.D. 562, the system of production, distribution and consumption of Sogaya potteries collapse completely.
In terms of tomb style of Sogaya, wooden coffin tombs are built during the 4th and the early 5th century, which are replaced by stone coffin tombs by the middle of the 5th century. Then, beginning in the 6th century, stone chamber tombs began to spread widely, starting with the upper class people.
No detailed researches have been done on the inner structure of Sogaya. Distribution of potter and burial type suggests that it was not a single political entity located in a specific location. Instead, it could be understood as a union of individual small states that were dispersed in the mid to bottom area of Nam-gang River and the southern coastal area. Since these small states of Sogaya exhibit significant homogeneity from archeological point of view in terms of pottery and burial type, it is thought that they maintained close relationship in terms of geography, blood ties, politics, economics and culture.
It could be speculated that, in Sogaya, a power center emerged in several regions first to form a federation, a horizontal political structure. Then, in each region, vertical political structure were developed. Central kofun group in Sogaya proves the existence of euplak (읍락, 國邑), a powerful polity that played a pivotal role as a center of politics. There were many euplak under gookeup (국읍) as its member. Also, euplak developed a vertical political structure consisting of smaller scale of chon (촌, 村).
The greater territory of Sogaya was defined by southern coastal area region and Nam-gang River area. The central polity includes tombs group in Songhak-dong in Goseong, Joongchon-li in Sancheong and Samga in Hapcheon. It is possible to postulate a multiple polities consisting of central and smaller villages in the scale of euplak.
It is speculated that Sogaya responded to their more powerful neighboring states by forming a collaborative network of central and smaller forces within their influence.
Based on the scale of kofun group, people who built kofuns in Joongchon-li in Sancheong was the earliest leader of Sogaya states, followed by people who built Samga kofun group. People who built Songhak-dong tombs in Goseong is considered a leader of Sogaya from the late 5th to early 6th century. However, the leaders of Sogaya did not appear to have power enough to dominate other states in terms of the strength of federation and their leadership in the federation.
It is speculated that Sogaya rapidly grew as it traded with other Gaya states, Baekje and Japan beginning in the late 5th century, which is proven by the distribution of potteries of Sogaya. Sogaya potteries are being unearthed not only in the eastern part of Jeollanam-do and Youngsan-gang River but also in Chungcheong-do area and Seoul in Baekjae period. In addition, it was verified that they were also associated Japanese potteries in most areas.
After the mid-6th century, Baekja and Shilla began to encroach upon Gaya region, which led to the fall of Sogaya. The influence of Daegaya and Baekjae expanded and the territorial expansion of Shilla in the southern coastal region began. Baekjae, after occupying Seomjin-gang River area in the early 6th century, began to invade Gaya area in full scale. Baekjae type stone chambers began to emerge in the northern area, which influenced Sancheong region too. In particular, in Jinju area, many Baekjae relics were unearthed, which makes us speculate that Baekjae used Jinju as the base for invading Sogaya region.
With the invasion of Shilla, Sogaya states in the southern coastal area collapsed and the region was reorganized. From the age of the tomb 1C in Songhak-dong in Goseong, the last of the tomb to be built, and the stone chamber tombs of Shilla, the fall of Sogaya is speculated to be around A.D. 560.