In the last stages of the bronze culture of the Karasuk affinity, the impact of the iron culture was experienced by ancient Koreans as a consequence of the rise of Chinese state power. The rise of Puyo was seen in Manchuria along with China's developing centralized power. In the southern part of Korea, tribal leagues of the Three Han gradually developed to the stage of state building. Paekche and Shilla were prominent in the south, Koguryo in the north.
By the first century, Koguryo was firmly established as a state power and destroyed the Chinese colony Lolang (Nangnang) in 313. In 342, however, Koguryo's capital fell to the Chinese Yen. Paekche amassed power while Koguryo was fighting against the Chinese, and came into conflict with Koguryo in the late fourth century. Then came the growth of Shilla with a more fully organized state power.
Koguryo was the first to adopt Buddhism as the royal creed in 372; Paekche, the second in 384; and Shilla, the last in 528. Buddhist scriptures in Chinese translation were also adopted. Koguryo established an academy to educate the nobility and compiled a state history consisting of 100 volumes before the introduction of Buddhism.. Paekche also compiled its history in the early fourth century prior to 384. Only Shilla undertood compilation of its history immediately following the adoption of Buddhism.
Thus, all Three Kingdoms developed highly sophisticated state organizations on the Korean Peninsula, adopting Confucian and Buddhist hierarchical structures with the king at the pinnacle. State codes were promulgated to initiate a legal system to rule the people. In this process, Koguryo annexed Puyo, and Shilla conquered Kaya. The Three Kingdoms were competing with each other in strengthening Buddhist-Confucian state power, in effort toward serious territorial expansion.
At this juncture, Shilla developed its Hwarang (Flower of Youth Corps), a voluntary military organization. The Hwarang members were trained as a group in the arts of war, literary taste and community life, partly through pilgrimages. The educational objectives were: 1) loyalty to the monarch, 2) filial piety to parents, 3) amicability among friends, 4) no retreat in war, and 5) aversion to unnecessary killing. These objectives were postulated by the famous monk Won-gwang, who consolidated Buddhist-Confucian virtues in the education of Shilla youths. This movement became popular and the corps contributed to the strength of the Shilla Kingdom.
With the youth corps, Shilla was able to amass state power in the cultural sphere as well. With the aid of a Paekche architect, it erected a huge temple, Hwangnyongsa ("Temple of the Illustrious Dragon"), and a towering pagoda famous even in China. The 70 meter high pagoda of Hwangnyongsa stood from 645 until the Mongol invasion of the 13th century. Shilla was ready to learn from Koguryo and Paekche and also dispatched monks to China to learn about China's culture, especially Chinese Buddhist doctrine, architecture and Chinese classics.
While Shilla was building amicable relations with Tang China, Koguryo was in fierce conflict with Sui and Tang. Sui Emperor Yang-ti, after successful campaigns against the northern nomadic tribes, invaded Koguryo with more than one million troops. In 612, Koguryo General Ulchi Mundok held the fortresses agains Tang0ti's army and navy for several months and destroyed the Sui troops in retreat. An ambush at Salsu (Ch'oongch'don'gang) river allowed only 2,700 Sui troops out of 300,000 men to escape. Sui fell from power partly as a result of the defeat by Koguryo.
After the rise of Tang, Tai-tsung contemplated revenge while protecting against invasion by building fortifications and walls along the Liao River. In 644, 648 and 655, Tai-tsung attempted unsuccessful invasions. Tang then turned to Shilla for assistance.
Shilla also persuaded Tang China to come to its aid in the conquest of Paekche and Koguryo. Koguryo had earlier defeated Sui Yang-ti, and Tai-tsung's hostile relationship drove Tang Kao-tsung to ally itself with Shilla in the campaign against Paekche and then Koguryo.
A late-comer to statehood, Shilla was finally able to defeat the other two kingdoms, but was unable to control the whole territory of Koguryo which extended to Manchuria. Tang's intention toward Shilla was made clear in the aftermath of the unification by Shilla. The Paekche king and his family were taken to Tang in 660 and a Tang general appointed a military governor to rule the Paekche territory. Koguryo's last king, his officials and 200,000 prisoners were also taken to China in 668 and Koguryo's territory was administered by Tang generals. Tang Kao-tsung's desires were now evident, and Shilla was determined to fight against Tang. The determination of Kim Yu-shin, Shilla's foremost general who led and marshaled Shilla's campaigns, counteracted the Chinese instigation of Paekche and Koguryo to rebel against Shilla. Shilla commenced active resistance against Chinese domination in Tang-controlled territory. In 671, Shilla started its own operations against Chinese rule and took the Chinese administrative headquarters, thereby retaking all of the Paekche territory. China invaded again in 674 against Shilla, who had succeeded in quelling the Tang army at Maech'o Fortress near Yanggu and the Ch'ionsong fortress at the Yesonggang river near Kaesong. Shilla's army also successfully drove out the Tang army from P'yongtang. Nevertheless, the Chinese army persistently claimed the territories of Paekche and Koguryo until 676 when they gave in to Shilla's claim of territory south of the Taedonggang river. Shilla became a unique state covering most of the Korean Peninsula and the majority of the people of the former three states.
One Koguryo warrior, Ko Sagye, who was taken by a Tang general, joined the Tang army. His son Ko Son-ji had a successful military career in Tang and conquered Tashkent in the mid-eighth century, transmitting paper-making technology to the Arabian countries. The Shilla monk Hyech'o in 727 visited India for pilgrimages to historic Buddhist sites in five Indian kingdoms, an account of which is preserved as an important historical record about eighth century India.
