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 Buyeo 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. Baekje and Silla were prominent in the south, Goguryeo in the north.

By the first century, Goguryeo was firmly established as a state power and destroyed the Chinese colony Lolang (Nangnang) in 313. In 342, however, Goguryeo's capital fell to the Chinese Yen. Baekje amassed power while Goguryeo was fighting against the Chinese, and came into conflict with Goguryeo in the late fourth century. Then came the growth of Silla with a more fully organized state power.

Goguryeo was the first to adopt Buddhism as the royal creed in 372; Baekje, the second in 384; and Silla, the last in 528. Buddhist scriptures in Chinese translation were also adopted. Goguryeo established an academy to educate the nobility and compiled a state history consisting of 100 volumes before the introduction of Buddhism. Baekje also compiled its history in the early fourth century prior to 384. Only Silla undertook 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, Goguryeo annexed Buyeo, and Silla conquered Gaya. The Three Kingdoms were competing with each other in strengthening Buddhist-Confucian state power, in efforts toward serious territorial expansion.

At this juncture, Silla 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 Silla youths. This movement became popular and the corps contributed to the strength of the Silla Kingdom.

With the youth corps, Silla was able to amass state power in the cultural sphere as well. With the aid of a Baekje 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 Hwang-nyongsa stood from 645 until the Mongol invasion of the 13th century. Silla was ready to learn from Goguryeo and Baekje, and also dispatched monks to China to learn about China's culture, especially Chinese Buddhist doctrine, architecture and Chinese classics.

While Silla was building amicable relations with Tang China, Goguryeo was in fierce conflict with Sui and Tang. Sui Emperor Yang-ti, after successful campaigns against the northern nomadic tribes, invaded Goguryeo with more than one million troops. In 612 Goguryeo General Eulji Mundeok held the fortresses against Yang-ti's army and navy for several months and destroyed the Sui troops in retreat. An ambush at Salsu (Cheongcheon-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 Goguryeo.

A hunting scene from a sixth century Goguryeo tomb.

The brick chamber of the tomb of King Muryeong (r. 501-523) of the Baekje Kingdom (18 B.C-A.D. 660)

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 Silla for assistance.

Silla also persuaded Tang China to come to its aid in the conquest of Baekje and Goguryeo. Goguryeo had earlier defeated Sui Yang-ti, and Tai-tsung's hostile relationship drove Kao-tsung of Tang to go into alliance with Silla in the campaign against Baekje and then Goguryeo.

A late-comer to statehood, Silla was finally able to defeat the other two kingdoms, but was unable to control the whole territory of Goguryeo which extended to Manchuria. Tang's intention toward Silla was made clear in the aftermath of the unification by Silla. The Baekje king and his family were taken to Tang in 660 and a Tang general appointed a military governor to rule the Baekje territory. Goguryeo's last king, his officials and 200,000 prisoners were also taken to China in 668 and Goguryeo's territory was administered by Tang generals. Kao-tsung's desires were now evident, and Silla was determined to fight against Tang. The determination of Kim Yu-sin, Silla's foremost general who led and marshaled Silla's campaigns, counteracted the Chinese instigation of Baekje and Goguryeo to rebel against Silla. Silla commenced active resistance against Chinese domination in Tang-controlled territory. In 671 Silla started its own operations against Chinese rule and took the Chinese administrative headquarters, thereby retaking all of the Baekje territory. China invaded again in 674 against Silla, who had succeeded in quelling the Tang army at Maecho Fortress near Yanggu and the Cheonseong fortress at the Yeseonggang river near Gaeseong. Silla's army also successfully drove out the Tang army from Pyongyang. Nevertheless, the Chinese army persistently claimed the territories of Baekje and Goguryeo until 676 when they gave in to Silla's claim of territory south of the Daedonggang river. Silla became a unique state covering most of the Korean Peninsula and the majority of the people of the former three states.

One Goguryeo warrior, Go Sa-gye, who was taken by a Tang general, joined the Tang army. His son Go Seon-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 Silla monk Hyecho 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.