The Silla Kingdom (57 BC - 936 AD) was the first to develop. It was formed on the southeastern Kyongju plain by a confederation of six clans of the Chin-han. The tribal states in the southwest united, calling their centralized kingdom Paekche (18 BC - 600 AD). Some of the tribal states in the area of the lower Naktong River, along the south central coast of the peninsula, did not join either of these kingdoms. They formed a league of walled city-states, called Kaya, which conducted extensive coastal trade and also maintained close ties with the tribal states in western Japan. Sandwiched between the more powerful Silla and Paekche, Kaya eventually was absorbed by its neighbors during the sixth century. The northern kingdom of Koguryo (37 BC - 668 AD) emerged from among the indigenous people along the banks of the northern Yalu River. It formed after Silla and before Paekche and was the largest of the three. Koguryo, Paekche, Silla formed the three kingdoms of ancient Korea.
The development of the Korean nation progressed through five distinct dynasties: Koguryo, Paekche, Silla, Koryo, and Joseon . The first three of these dynasties existed simultaneously. This time is known as the Three Kingdoms Era.
These kingdoms evolved into three distinct entities. In fact, each had its own dialect of the developing Korean language. The three kingdoms rivaled among themselves for power as each grew in national strength, so conflict was inevitable. Confrontations between the three kingdoms may be identified in three stages:
The first stage covers the period from the reign of Koguryo's King Srurim to the reign of King Munja, in which Koguryo became the aggressor. Koguryo looked to expand its borders toward the southern kingdoms of Silla and Paekche to gain land that was more fertile and to strengthen its position against the menacing Hsien Pei and other violent, northern nomadic tribes.
The second stage was marked by Silla's build up of power until it was strong enough to overcome Koguryo's dominance. The creation of the Hwarang warriors, by King Chin-Hung, was the main reason for Silla's military success.
The final stage began in 589 AD when Sui unified China and undertook a massive invasion, with over one million men, against Koguryo. Sui underestimated the powerful resistance of the Koguryo people, who inflicted such severe defeats upon the Chinese that it caused the downfall of the Sui Dynasty and so founded the T'ang Dynasty (618-904 AD), one of China's most glorious dynasties. The T'ang also launched attacks against Koguryo, which Koguryo withstood while she pitted herself against the other two kingdoms, who were also fighting against each other. Silla formed an alliance with T'ang China, conquered Paekche, and finally conquered Koguryo, ending the wars and unifying all of Korea under the Silla Kingdom. This marked the beginning of the Silla Dynasty (661-935 AD) and led to the rapid development of Korea.