57 BC – 668 AD
The Three Kingdoms that dominated Korea and part of Manchuria during the 1st millennium were Goguryeo (37 BC ~ 668 AD), Baekje (18 BD ~ 660 AD), and Silla (57 BC ~ 935 AD). The three kingdoms spanned the entire Korean peninsula, as well as Manchuria. They shared similar cultures, languages, religions, as well as influence from the Chinese. With the help of the Tang dynasty of China, Silla was able to unify the Korean peninsula in the 7th century. This marks the first time in history that the Korean Peninsula was ever unified. Prior to Silla unifying the Korean Peninsula, people living in the three kingdoms did not share ethnic identities with each other, despite their similar cultural origins.
In terms of religion, all three kingdoms originally had their own shamanistic religions, but as Chinese cultures were brought in, Confucianism and Daoism flourished until Buddhism was introduced by Chinese missionaries into the peninsula. Buddhism was so widely accepted that it eventually became the official religion of the three kingdoms.
After the fall of Gojoseon, the Goguryeo kingdom was established on both north and south banks of the Yalu River. It is thought to be the first of the three kingdoms to be established, as well as the most advanced. The Goguryeo kingdom did eventually become the largest of the three kingdoms. Their conquest of the northern Korean peninsula and Manchuria allowed them controll many Chinese and Tungusic tribes located in those areas as well. The Goguryeo kingdom was finally overthrown in the year 668 by an allied force of the Chinese Tang dynasty and the Korean Silla kingdom.
The Baekje kingdom was established by two sons of the Goguryeo kingdom. The two sons sought to establish a new kingdom around present day Seoul after fleeing a succession conflict in the Goguryeo kingdom. It was initially established as a kingdom under the Mahan confederacy. After conquering many Mahan chiefdoms, the Baekje kingdom controlled the majority of the western Korean peninsula by the 4th century. The Tamna kingdom of Jeju Island also became a tributary of the Baekje kingdom.
The Silla kingdom controlled the southeast Korean peninsula. It was the last founded of the three kingdoms. The Silla kingdom sought to ally with the Chinese Tand Dynasty in order to conquer the Goguryeo and Baekje alliance. After successfully overthrowing both Goguryeo and Baekje, the Silla kingdom controlled the Korean peninsula south of Pyongyang; whereas the Tang forces were driven out of the Korean peninsula. Due to its close relations with the Tang dynasty, the Silla kingdom received much influence of Chinese culture and religion.