The Kingdom fell apart during the establishment of the regional confederation, but the factions that founded Goguryeo and Baekje took pride in their status as the inheritors of Buyeo. Samguk sagi (History of the Three Kingdoms) states that Gojumong, who founded Goguryeo in B.C.37, was originally from Buyeo. Goguryeo prospered greatly in areas close to Baekdusan Mountain and along the Amnokgang (Yalu) River. Right after its foundation, the Kingdom conquered a number of small states in the area and moved its capital to Gungnaeseong (Tonggu) near the Amnok. Through many wars, it drove away the factions loyal to Han Dynasty and expanded its territory as far as Liaodong in the west and to the north of the Korean Peninsula in the east. It became a powerful state, exerting control over Manchuria and the northern part of the Korean Peninsula.
There were also a number of small states, such as Okjeo and Dongye,in present-day Hamgyeong-do Province and the north of Gangwon-do Province along the East Coast of the Korean Peninsula. Located in outlying areas, they did not develop very rapidly. Okjeo offered tributes, such as salt and fish, to Goguryeo. The people of Dongye held a sacrificial rite for heaven called Mucheon in October, building a spirit of collaboration by singing and dancing together. Their specialty products included an archery bow, known as a dangung, and the gwahama (a horse small enough to pass unhindered beneath fruit trees). These two states were also incorporated into Goguryeo.
The area to the south of Gojoseon was occupied by a large group of small states including Mahan, Jinhan, and Byeonhan. Mahan was a confederacy of fifty-four small states (composed of 100,000 households in total) located in present-day Gyeonggi-do, Chungcheong-do, and Jeolla-do Provinces. Byeonhan was located in an area that included present-day Gimhae and Masan. Jinhan was located in an area that included present-day Daegu and Gyeongju. Each of the latter two was composed of 40,000 – 50,000 households. The three mini-states were collectively known as Samhan (Three Han States). The people of Samhan held rites of sacrifice for heaven in May and October. On such occasions, they gathered together to enjoy liquor, food, singing and dancing.
Along with the spread of iron culture and the development of farming skills, powerful states such as Goguryeo, Baekje and Silla gradually became established in Manchuria and the Korean Peninsula.
Goguryeo was the first of the three Kingdoms to firmly establish itself as a sovereign country. It started expanding its territory in the late 1st century and adopted a system centered on the King by the late 2nd century. By the early 4th century, King Micheon of Goguryeo had driven away factions loyal to Han Dynasty from the Korean Peninsula.
In 372 (the 2nd year of King Sosurim’s reign), Goguryeo adopted Buddhism and announced a code of laws in an effort to establish a proper ruling system. It also established the Taehak, a Confucian educational institute. King Gwanggaeto the Great, a son of King Sosurim, drove away the Khitan, Sushen, Dongbuyeo and expanded his territory into Manchuria. He also captured many of Baekje’s fortresses in the south and helped Silla overcome a crisis by driving away Wako invaders.
Baekje was established in B.C.18 jointly by the people who lived along the Hangang River, people originating from Buyeo and Goguryeo, and migrants from elsewhere. By the mid-3rd century, during the reign of King Goi, the Kingdom had seized complete control over the areas along the Hangang River and established a solid system of political governance by accommodating the advanced culture of China. By the mid-4th century, King Geunchogo occupied Mahan and expanded the territory as far as the south coast of present-day Jeollanam-do Province. Along the northern border, Baekje confronted Goguryeo in a bid to take control of present-day Hwanghae-do Province. It also exerted control over Gaya in the south. At that time, Baekje’s territory included present-day Gyeonggi-do Province, Chungcheong-do Province, Jeolla-do Province, the middle reaches of the Nakdonggang River, Gangwon-do Province, and Hwanghae-do Province.
Silla originated in Saroguk, one of the mini states of Jinhan. It was established as a Kingdom in B.C.57 by the natives of present-day Gyeongju and people from other regions. Those with the family names Park, Seok, and Kim acceded to the throne in turn. By around the 4th century, the Kingdom occupied most of the areas east of the Nakdonggang River. During the reign of King Naemul, Silla allowed Goguryeo troops to remain within the Kingdom to help drive away Wako invaders. It also adopted Chinese culture and civilization through Goguryeo.
In Byeonhan, located along the lower reaches of the Nakdonggang River, the Gaya Confederation emerged, with Geumgwan Gaya playing a leading role. The confederation developed an iron culture and exerted considerable influence on areas along the Nakdonggang River. Mini states of Gaya started rice farming early on and traded actively with Wa (Japan) and Lelang, taking advantage of locally produced iron and convenient sea routes.
Unification of the Three Kingdoms under Silla
By the 5th century, each of the three Kingdoms (Goguryeo, Baekje, and Silla) on the Korean Peninsula was committed to a policy of territorial expansion under a firmly established ruling apparatus centered on the King. In Goguryeo, King Jangsu, a son of King Gwanggaeto, moved the capital to Pyeongyang 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, Gyeonggi-do Province. Thanks to this territorial expansion, Goguryeo emerged as a power to be reckoned with in Northeast Asia.
Looking at Baekje, the Kingdom moved its capital to Ungjin (present-day Gongju) in 475, after yielding the areas along the Hangang River to Goguryeo. It strived to rebuild its strength to regain the lost territory. King Dongseong confronted Goguryeo by reinforcing the alliance with Silla. King Muryeong reinforced local control in an effort to lay the foundation for prosperity. King Seong, a son of King Muryeong, relocated the Baekje capital to Sabi (presentday 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 its administrative zones, including the capital, during the reign of King Jijeung. King Jijeung incorporated Usanguk (composed of present-day Ulleungdo and Dokdo) into Unified Silla and Balhae (8th Century) the territory of Silla in 512. King Beopheung stabilized the ruling system by proclaiming laws, setting rules about official robes, and adopting Buddhism as the official state religion. He also incorporated Geumgwan Gaya in a drive to expand the territory. King Jinheung reorganized Hwarangdo into a national organization and expanded the territory considerably. He seized lands along the Hangang River from Baekje, conquered Dae Gaya in Goryeong, wrested areas along the Nakdonggang River, and expanded the territory as far as Hamheung along the East Coast.
In 612, Sui 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 (present-day Cheongcheongang River). 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, but failed 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. 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.