By the 5th century, each of the three Kingdoms (Goguryeo, Baekje, and Silla) on the Korean Peninsula was committed to a policy for territorial expansion under a firmly established governing apparatus centered on the King. In Goguryeo, King Jangsu, a son of King Gwanggaeto, moved the capital to Pyongyang in 427. He occupied Hanseong (present-day Seoul), the capital of Baekje, and areas along the Hangang River, expanding his territory down to present-day Jungnyeong Pass (Danyang and Yeongju) and Namyang-myeon, Gyeonggido. Thanks to this territorial expansion, Goguryeo established a colossal empire in Manchuria and on the Korean Peninsula, dominating as a power in Northeast Asia.

After yielding the areas along the Hangang River to Goguryeo, Baekje moved its capital to Ungjin (present-day Gongju) in 475. It strived to rebuild its strength to regain the lost territory. King Dongseong confronted Goguryeo by reinforcing the alliance with Silla. King Muryeong strengthened local control in order to lay a solid foundation for prosperity. King Seong, a son of King Muryeong, relocated the Baekje capital to Sabi (present-day Buyeo), strove to reform the ruling system, and regained control over areas along the Hangang River in an alliance with Silla.

As for Silla, Saroguk changed its name to Silla in the early 6th century, reformed its political system, and reorganized the administrative zones, including the capital, during the reign of King Jijeung. King Jijeung incorporated Usanguk (composed of present-day Ulleungdo and Dokdo Islands) into the territory of Silla in 512. King Beopheung stabilized the ruling system as a centralized state by promulgating a code of law, setting up rules about official robes, and adopting Buddhism as the official state religion. He also incorporated Geumgwan Gaya in a drive to expand his territory. King Jinheung reorganized the Hwarangdo into a national organization and expanded the territory considerably. He seized lands along the Hangang River from Baekje, conquered Daegaya in Goryeong, wrested areas along the Nakdonggang River, and expanded the territory as far as to Hamheung along the east coast.

In 612, the Sui dynasty in China, which unified all of mainland China into one state, attacked Goguryeo, mobilizing more than a million troops. General Eulji Mundeok of Goguryeo drowned most of the Chinese invaders in the Salsu River (present-day Cheongcheongang River), which is called the Battle of Salsu. The Sui dynasty sustained enormous damage due to the failure of the campaign and fell to the Tang dynasty in 618. Tang China also attacked Goguryeo several times butfailed at each attempt.

In the meantime, Baekje frequently attacked Silla. Silla unsuccessfully sought the assistance of Goguryeo, and then invaded in an alliance with Tang China. Silla troops led by Kim Yu-sin defeated an elite force of Baekje troops commanded by Gyebaek in Hwangsanbeol and marched to Sabi, the capital of Baekje. Troops of Tang China invaded Baekje through the estuary of the Geumgang River. Finally, Baekje surrendered to the Silla-Tang forces in 660.

The Silla-Tang forces then attacked Goguryeo, once the most powerful Kingdom in Northeast Asia. However, Goguryeo had depleted its resources in two large-scale wars against the two dynasties of China, and fell in 668.

Upon conquering Baekje and Goguryeo in alliance with Silla, Tang China attempted to exert control over the entire Korean Peninsula including Silla by establishing the Ungjin Commandery in Baekje, the Protectorate-General to Pacify the East in Goguryeo, and the Gyerim Territory Area Command even in Silla. Silla waged a war against Tang, defeated its navy in Gibeolpo near the estuary of the Geumgang River, and drove all of Tang’s forces out of the peninsula, thus accomplishing the important feat of unifying the Korean Peninsula in 676.