In 540 AD, after King Chin Hung had assumed power in Silla, came many years of wars between Silla and Koguryo. However, before Koguryo was engaged, King Chin Hung took on the task of driving the Japanese colonies in the south out of Silla. At about this same time, a small Kaya league had formed in the area between Paekche and Silla that had strong ties with the Wa State of Japan. King Chin Hung annexed the portion of the Kaya league that was in alliance with the Yamato Clan of Japan.
King Chin Hung then turned his armies toward the fertile valleys in central Korea, between the Han and Imjin Rivers. The conquest of Han-Imjin River area brought great wealth to Silla through the acquisition of the richest agricultural lands in the peninsula and of its peasantry labor. This conquest also opened an easier route to China through the capture of ports on the Yellow Sea. The acquisition of iron mining regions led to new technological gains that predicated even greater expansion by Silla.
King Chin Hung asked the famous Buddhist priest, Wan Kwang Bopsa, to develop a system of martial arts that was in harmony with his concepts of the laws of nature. The result was the creation of the Hwarang warriors who played a crucial role Silla's effort to unify Korea. The Hwarang are discussed in detail in a later section of this topic.