Baekje was founded by Onjo, the third son of Goguryeo's founder Jumong and So Seo-no, at Wiryeseong (present-day southern Seoul). Baekje, like Goguryeo, claimed to succeed Buyeo, a state established in present-day Manchuria around the time of Gojoseon's fall.
Baekje alternately battled and allied with Goguryeo and Silla as the three kingdoms expanded control over the peninsula. At its peak in the 4th century, Baekje controlled most of the western Korean peninsula, as far north as Pyongyang, and may have even held territories in China, such as in Liaoxi, though this view is controversial. It became a significant regional sea power, with political and trade relations with China and Japan.
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 was founded in 18 BC by King Onjo, who led a group of people from Goguryeo south to the Han River basin. According to the Chinese Records of the Three Kingdoms, during the Samhan period, one of the chiefdoms of the Mahan confederacy was called Baekje.
The Samguk Sagi provides a detailed account of Baekje's founding. Jumong had left his son Yuri in Buyeo when he left that kingdom to establish the new kingdom of Goguryeo. Jumong became Divine King Dongmyeong, and had two more sons with So Seo-no, Onjo and Biryu. When Yuri later arrived in Goguryeo, Jumong promptly made him the crown prince. Realizing Yuri would become the next king, So Seo-no left Goguryeo, taking her two sons Biryu and Onjo south to found their own kingdoms with their people, along with ten vassals. She is remembered as a key figure in the founding of both Goguryeo and Baekje.