Goguryeo
Goguryeo was founded in 37 BCE by Jumong (posthumously titled as Dongmyeongseong, a royal given title). Later, King Taejo centralized the government. Goguryeo was the first Korean kingdom to adopt Buddhism as the state religion in 372, in King Sosurim's reign.
Goguryeo (also spelled as Koguryŏ) was also known as Goryeo (also spelled as Koryŏ), and it eventually became the source of the modern name of Korea.
The 3rd and 4th centuries were characterized by territorial competition with the Chinese and Xianbei, resulting in both losses and gains. Goguryeo initiated the Goguryeo–Wei War by attacking a Chinese fortress in 242 in an attempt to cut off Chinese access to its territories in Korea. Cao Wei of the Three Kingdoms of China retaliated by invading and destroying Hwando in 244. This forced the king to flee with Cao Wei in pursuit and broke Goguryeo's rule over the Okjeo and Ye, damaging its economy. The king eventually settled in a new capital, and Goguryeo focused on rebuilding and regaining control. In the early 4th century Goguryeo once again attacked the Chinese (now Sima Jin) to cut off their access to Korea and this time succeeded, and soon afterward conquered Lelang and Daifang ending the Chinese presence in Korea. However Goguryeo's expansion led to confrontation with the rising Xianbeis. The Xianbeis devastated Goguryeo's capital in the mid 4th century and the king retreated. Goguryeo eventually regrouped and began striking back in the late 4th century by King Gogukyang, culminating with the conquests of Gwanggaeto the Great.
Goguryeo reached its zenith in the 5th century, becoming a powerful empire and one of the great powers in East Asia, when Gwanggaeto the Great and his son, Jangsu, expanded the country into almost all of Manchuria, parts of Inner Mongolia, parts of Russia, and took the present-day city of Seoul from Baekje. Goguryeo experienced a golden age under Gwanggaeto and Jangsu, who both subdued Baekje and Silla during their times, achieving a brief unification of the Three Kingdoms of Korea and becoming the most dominant power of the Korean peninsula. Jangsu's long reign of 79 years saw the perfecting of Goguryeo's political, economic and other institutional arrangements.
Goguryeo was a highly militaristic state; in addition to contesting for control of the Korean Peninsula, Goguryeo had many military conflicts with various Chinese dynasties, most notably the Goguryeo–Sui War, in which Goguryeo defeated a huge force traditionally said to number over a million men, and contributed to the Sui dynasty's fall.
In 642, the powerful general Yeon Gaesomun led a coup and gained complete control over Goguryeo. In response, Emperor Tang Taizong of China led a campaign against Goguryeo, but was defeated and retreated. After the death of Tang Taizong, his son Emperor Tang Gaozong allied with the Korean kingdom of Silla and invaded Goguryeo again, but was unable to overcome Goguryeo's stalwart defenses and was defeated in 662. However, Yeon Gaesomun died of a natural cause in 666 and Goguryeo was thrown into chaos and weakened by a succession struggle among his sons and younger brother, with his eldest son defecting to Tang and his younger brother defecting to Silla. The Tang–Silla alliance mounted a fresh invasion in 667, aided by the defector Yeon Namsaeng, and was finally able to conquer Goguryeo in 668.
After the collapse of Goguryeo, Tang and Silla ended their alliance and fought over control of the Korean Peninsula. Silla succeeded in gaining control over most of the Korean Peninsula, while Tang gained control over Goguryeo's northern territories. However, 30 years after the fall of Goguryeo, a Goguryeo general by the name of Dae Joyeong founded the Korean-Mohe state of Balhae and successfully expelled the Tang presence from much of the former Goguryeo territories.
Baekje
Baekje was founded by Onjo, a Goguryeo prince and the third son of the founder of Goguryeo, in 18 BCE. Baekje and Goguryeo shared founding myths and originated from Buyeo. The Sanguo Zhi mentions Baekje as a member of the Mahan confederacy in the Han River basin (near present-day Seoul). It expanded into the southwest (Chungcheong and Jeolla provinces) of the peninsula and became a significant political and military power. In the process, Baekje came into fierce confrontation with Goguryeo and the Chinese commanderies in the vicinity of its territorial ambitions.
At its peak in the 4th century during the reign of King Geunchogo, Baekje absorbed all of the Mahan states and subjugated most of the western Korean peninsula (including the modern provinces of Gyeonggi, Chungcheong, and Jeolla, as well as part of Hwanghae and Gangwon) to a centralized government. Baekje acquired Chinese culture and technology through maritime contacts with the Southern dynasties during the expansion of its territory.
Baekje was a great maritime power; its nautical skill, which made it the Phoenicia of East Asia, was instrumental in the dissemination of Buddhism throughout East Asia and continental culture to Japan. Baekje played a fundamental role in transmitting cultural developments, such as Chinese characters, Buddhism, iron-making, advanced pottery, and ceremonial burial to ancient Japan. Other aspects of culture were also transmitted when the Baekje court retreated to Japan after Baekje was conquered by the Silla–Tang alliance.
Baekje was once a great military power on the Korean Peninsula, especially during the time of Geunchogo, but was critically defeated by Gwanggaeto the Great and declined. Ultimately, Baekje was defeated by a coalition of Silla and Tang forces in 660.
Silla
According to legend, the kingdom of Silla began with the unification of six chiefdoms of the Jinhan confederacy by Bak Hyeokgeose in 57 BCE, in the southeastern area of Korea. Its territory included the present-day port city of Busan, and Silla later emerged as a sea power responsible for destroying Japanese pirates, especially during the Unified Silla period.
Silla artifacts, including unique gold metalwork, show influence from the northern nomadic steppes and Iranian peoples and especially Persians, with less Chinese influence than are shown by Goguryeo and Baekje. Silla expanded rapidly by occupying the Nakdong River basin and uniting the city-states.
By the 2nd century, Silla was a large state, occupying and influencing nearby city-states. Silla gained further power when it annexed the Gaya confederacy in 562. Silla often faced pressure from Goguryeo, Baekje and Japan, and at various times allied and warred with Baekje and Goguryeo.
Silla was the smallest and weakest of the Three Kingdoms of Korea, but it used cunning diplomatic means to make opportunistic pacts and alliances with the more powerful Korean kingdoms, and eventually Tang China, to its great advantage.
In 660, King Muyeol of Silla ordered his armies to attack Baekje. General Kim Yu-shin, aided by Tang forces, conquered Baekje. In 661, Silla and Tang moved on Goguryeo but were repelled. King Munmu, son of Muyeol and nephew of Kim, launched another campaign in 667 and Goguryeo fell in the following year.
Gaya
Gaya was a confederacy of small kingdoms in the Nakdong River valley of southern Korea, growing out of the Byeonhan confederacy of the Samhan period. Gaya's plains were rich in iron, so export of iron tools was possible and agriculture flourished. In the early centuries, the Confederacy was led by Geumgwan Gaya in the Gimhae region. However, its leading power changed to Daegaya in the Goryeong region after the 5th century.
Constantly engaged in war with the three kingdoms surrounding it, Gaya was not developed to form a unified state, and was ultimately absorbed into Silla in 562.