Koguryo:Of the Three Kingdoms, Koguryo was the strongest. It developed in Manchuria and the northern part of the Korean Peninsula, and was founded by King Chujmong in the latter half of the 2nd century. As the first nation of an Ancient Kingdom, Koguryo grew independently while fighting Chinese domination. By the 4th century under the reign of King Mich'on, Koguryo occupied Nangang Country, and took back almost all of the former territory Kojoson (313). In the mid-4th century, as results from the invasions by Paekche, Koguryo suffered a great loss. When King Sosurim was in reign, Koguryo accepted Buddhism and established a school called T'aehak (372). He also readjusted Koguryo's national structure. During the reign of King Kwanggaet'o the Greatest and King Changsu, Koguryo expanded its territory vastly, and held hegemongy in northeastern Asia. Under the reign of Kwanggaet'o the Greatest, Koguryo conquered Manchurian territory to the east of Liaohe, attacked Pakeche to occupy the north of the Han River, and expelled the foreign foes who were trying to invade Shilla. In the 5th century, King Changsu, moved its capital from Kungnaesong to P'yong'yang (427). Koguryo became a powerful country that ruled over the middle section of the south Hangang River basin by attacking Paekche.

Paekche: Paekche developed along the Hangang River basin and the plains of Kyongju. The founder of Paekche was King Onjo. In the mid-4th century, during the reign of King Kunch'ogowang, Paekche progressed into a Golden Age. King Kunch'ogowang conquered the remaining land of Mahan, completely unified the Hongam district (now called Cholla-do), and by rising victorious from battles with Koguryo, ruled over the Hwanghae-do. During this period, Paekche also made inroads into Japan and parts of China. Including Shandong and Liaoxi. In the 5th century, Paekche confronted the southward advancement of Koguryo. Paekche lost the territory along the Hangang River basin and moved its capital to Ungjin (Kongj). Weakened, Paekche in the 6th century, under the reign of King Songwang, moved its capital to Sabisong (Puyo), and began readjusting its institution to revive itself. King Songwang also promoted cultural exchanges with the Southern Dynasty of China and introduced Buddhism to Japan (384). Paekche also introduceed and taught Chinese study to Japan (405). During this period, Paekche joined Silla and recovered the lower Hangang River from Koguryo, only to lose it agan to Shilla afterwards.

Shilla: Saroguk, in Kyogju area, developed into the kingdom of Shilla by King Pak Hyokkose. It developed the slowest among the Three Kingdoms. In the 6th century, King Chijungwang changed the name of his country from Saroguk to Shilla (503). In 527 AD, King Pophunwag officially reconized Buddhism . And in 520 AD, regulations and promulgated laws were officially reconized. King Chinhungwang occupied the Hangang River and Naktonggang River basins and advaced into the Hamhung Plains.

Koguryo: When Shilla conquered the Han River basin, Koguryo and Paekche made a pact to attack Shilla. When this happened, Shilla went to China to request an alliance. At this time, Sui unified China. And because Koguryo was close to China, Sui frequently attacked Koguryo. Koguryo managed to expel them with resistance of its united army and entire population. When China attacked with a large army of over one million under the personal command of Emperor Yangdi of Sui, General Ulchimundok feigned defeat to lure the 300,000 special soldiers of Sui. He annihilated them at the Battle of Salsu (Ch'ongch'ngang RIver) in 612 AD. After the Sui Dynasty, the Tang Dynasty invaded Koguryo. In Koguryo, King Yongaesomun was in power and faced Tang's army. Koguryo drove T'aijong and his army in 645 AD. The attack lasted only sixty days.

Shilla: Since Shilla was isolated, the Tang Dynasty and Shilla formed an alliance. With the combined forces of China, Shilla attacked Paekche first. When Shilla came to Paekche territory, the suicide squad of Kyebaek confronted them under the command of Kim Yu-sin. They fought well but because Shilla and Tang had to many soldiers, they were defeated. Sabisong soon fell to the allied troops of Shilla and Tang. In 660 AD, Paekche was conquered.

Meanwhile, in Koguryo King Yongaesomun died, and internal dissension rose among the leading generals. Shilla and Tang took advantage of the opportunity and after a year of war, P'yong'yang fell. In 668 AD, Koguryo was also conquered. When Paekche and Koguryo were defeated, Tang ruled over their former territory. Later Tang tried to rule Shilla, but Shilla combined with the strength of the people of Paekche and Koguryo, waged war against Tang. After ten years, the combined forces of the Three Kingdoms defeated the Tang army in the Battle of Maech'song, and the Tang navy at the mouth of the Kumgang River. By driving the Tang armed forces out, Shilla unified the Three Kingdoms in 676 AD. But the unification wasn't a complete one, it was assisted by foreign forces and was limited to the area south of Taedongang River and the Wonsang Bay. In the Three Kingdoms, agriculture developed as the most important industry. In Paekche and Shilla, irrigation facilities expanded with rice farming. In Koguryo, millet soybeans, and other dry crops flourished.

After Shilla unified the Thee Kingdoms, Shilla readjusted its political systems and other ruling institutions. They also stabilized both the government and society, and the royal powers of the King were strengthened as result of the unbroken lineage from King Muyolwang. For the next hundred years Shilla prospered. But with the later half of the 8th century, hardships started. When this happened, impoverished peasants became either slaves or thieves. During the reign of Queen Chinsongyowang, peasants rebelled everywhere and civil wars were all over Shilla.