linguistic:voice_change, linguistic:paraphrasing, structural:expansion
Foundation

The Three Kingdoms were founded after the fall of Wiman Joseon and gradually conquered and absorbed various other small states and confederacies. After the fall of Gojoseon, the Han dynasty established four commanderies in the northwestern Korean Peninsula and present Liaoning. Three fell quickly to the Samhan, and the last was destroyed by Goguryeo in 313.

The nascent precursors of Baekje and Silla expanded within the web of statelets during the Proto-Three Kingdoms period, and Goguryeo conquered neighboring state like Buyeo in Manchuria and chiefdoms in Okjeo, Dongye which occupied the northeastern Korean Peninsula. The three polities made the transition from walled-town state to full-fledged state-level societies between 1st – 3rd century AD.

The primary sources for this period include Samguk sagi and Samguk yusa in Korea, and the "Eastern Barbarians" section (東夷傳) from the Book of Wei (魏書) of the Records of the Three Kingdoms in China.

All three kingdoms shared a similar culture and language. The Book of Sui (Volume 81) recorded: "The customs, laws and clothes of Goguryeo, Baekje and Silla are generally identical." Their original religions appear to have been shamanistic, but they were increasingly influenced by Chinese culture, particularly Confucianism and Taoism. In the 4th century, Buddhism was introduced to the peninsula and spread rapidly, briefly becoming the official religion of all three kingdoms.

According to Lisa Kay Bailey, the material culture of the Three Kingdoms can be clearly distinguished as they displayed cultural influence from different regions. Goguryeo's culture showed stronger influence from northern Chinese art, Baekche showed stronger influence from southern Chinese art, and Silla, which was more distant from China, showed greater influence from Eurasian steppe nomad cultures and greater preservation of native traditions. During this period, the Three Kingdoms had yet to unify their separate identities. Each kingdom produced their own individual histories; only in the Goryeo dynasty period was the collective history of the Korean Peninsula written together.

"The decline of Chinese power in the fourth century unleashed a wave of refugees that proved pivotal in speeding up the process of state-building in Korea," starting the Three Kingdoms era.