II. THE GROWTH OF THE THREE KINGDOMS AND UNIFICATION
1. The Rise and Development of the Three Kingdoms

(1) The Rise of the Three Kingdoms and Ancient Kingdoms

The patriarchal states which arose in various areas of the Korean peninsula and surrounding areas were gradually unified into the Three Kingdoms and continued to develop. Koguryo developed in Manchuria and the northern part of the Korean peninsula, and Paekche and Shilla arose along the Han River basin and the plains of Kyongju.
According to the records of the Samguk sagi (History of the Three Kingdoms), Koguryo was established by King Chumong (Tongmyong Songwang), Paekche by King Onjo, and Shilla by King Pak Hyokkose respectively.
The Three Kingdoms strengthened their monarchies, united patriarchal powers, expanded their territories and centralized their power. Thus, with the particular features of strong royal authority and a centralized governing structure they developed as ancient kingdoms.
 
(2) Development of Koguryo and Paekche
Koguryo was the first nation to develop as an ancient kingdom. After the downfall of Kojoson, Chinese influences permeated inward for a time, but Koguryo grew independently while fighting against Chinese domination.
In the latter half of the 1st century under King T'aejo's reign, Koguryo strengthened its base as an ancient kingdom and in the early 4th century during the reign of King Mich'on, it occupied Nangnang county and recovered almost all of the former territory of Kojoson.
On the other hand, along the Han River basins, many immigrants settled from the north at the time of Kojoson's downfall. These were chiefly the people of Puyo and Koguryo. The kingdom of Paekche was established with the immigrants as its center. Paekche, which first began as the small nation of Mahan, gradually gained power.
In the mid-3rd century during the reign of King Koi, Paekche concentrated its strength in the greater part of the Han River basin in order to solidify its base as an ancient kingdom, and in the mid-4th century during the reign of Kunch'ogo, it progressed into its Golden Age. King Kunch'ogo conquered the remaining land of Mahan to the south, completely unified the Honam district (now Cholla-do) and by rising victorious over battles with Koguryo, ruled over the Hwanghae Province to the north. During this period, Paekche also made inroads into Japan and parts of China including Shandong and Liaoxi.
In the mid-4th century, Koguryo suffered a great loss as a result of invasions by Former Yan and Paekche but in the latter half of the 4th century, during the reign of King Sosurim, Koguryo accepted Buddhism, established a school called T'aehak, and through the promulgation of various laws, readjusted its national structure. During the consecutive reigns of King Kwanggaet'o and King Changsu, Koguryo greatly expanded its territory and held hegemony in north-eastern Asia.
 
A 627 cm stele to King Kwanggaet'o in Tonggou,
Jianxian, China, constructed in A.D. 414.
 
Under the reign of King Kwanggaet'o, Koguryo occupied Manchurian territory to the east of Liaohe, attacked Paekche to occupy the north of the Han River and expelled foreign foes who were trying to invade Shilla. In the 5th century during the reign of King Changsu, Koguryo moved its capital from Kungnae-song to P'yong'yang and became a powerful country which ruled over the middle section of the south Han River basin by attacking Paekche.
Paekche, in the 5th century, confronted the southward advancement of Koguryo, lost the territory along the Han River basin and moved its capital to Ungjin (Kongju). With its power weakened, Paekche, in the 6th century under the reign of King Song, again moved its capital, this time to Sabisong (Puyo), and began readjusting its institutions in order to revive itself. King Song actively promoted cultural exchanges with the Southern Dynasties of China and introduced Buddhism to Japan. During this period, Paekche joined Shilla to help control the Han River, only to lose it again to Shilla.
 
(3) Development of Shilla and Changes in Kaya
 
A monument erected in honor of King Chinhung's expansion of Shilla's territory.
Located on Mt. Pukhan.
 
Saroguk, which arose in the area of Kyongju, developed into the kingdom of Shilla. From the 1st century, Shilla, under King Naemul, developed by combining the entities around it and by the latter 4th century, built the base for the ancient kingdom. Ever since King Naemul's era, successors to the throne were all members of the Kim family. The power of the king was further strengthened as he was referred to as "Maripkan", the Great Chief, instead of "Isagum", meaning successor.
At that time, Shilla, with the aid of King Kwanggaet'o of Koguryo, drove out Japanese invaders from its land. Afterwards, Shilla was under the influence of Koguryo for a time, but by concluding a treaty of alliance with Paekche, overcame the influence of Koguryo. On the other hand, in the lower delta of the Naktong River, a confederation of six Kaya states appeared with Kumgwan Kaya in Kimhae and Tae Kaya in Koryong as the centers. The power of Kaya was so strong that at one time it threatened Shilla and advanced toward Japan, but it was annexed by Shilla without having developed into an ancient kingdom.
.. Shilla, which developed slowest among the Three Kingdoms, had made great progress by the 6th century. King Chijung changed the name of his country to Shilla and adopted the title of King instead of Maripkan. Under the reign of King Pophung, Shilla officially recognized Buddhism and promulgated laws and regulations. King Chinhung occupied the Han and Naktong River basins and even made inroads into the Hamhung Plains.