Geunchogo, the second son of Biryu, the eleventh king of Baekje, had become king in 346 upon the death of the twelfth king, Gye.

His reign seems to have marked the permanent ascendancy of the descendants of the fifth king Chogo (reflected in Geunchogo's name) over those of the eighth king, Goi, and ended the alternating kingship of the two lines.

Having set set out to solidify the royal power within the Baekje state upon ascending the throne, he has reduced the power of the aristocracy and set up a system of local government with regional heads appointed by the court.

He has married a wife from the Jin clan, setting a precedent for his successors, and he has moved the capital to Hansan, today's southeast Seoul.

The first Korean envoy to Japan, an emissary of the government of Baekje, reportedly arrives in 366.

In this year, Geunchogo allies with Silla, which borders Baekje on the east, maintaining a rough balance of power among the Three Kingdoms.

Baekje reaches its greatest geographic extent and political power under Geunchogo, expanding Baekje's territory to the north through war against Goguryeo, while annexing the remaining Mahan societies in the south in 369, completing Baekje's control over all of present-day Jeolla-do.

Gaya confederacy states west of the Nakdong River were also made Baekje dependencies.

During Geunchogo's reign, the territories of Baekje include most of the western Korean Peninsula (except the two Pyeongan provinces).

Baekje is invaded by Goguryeo in 369, but counterattacks in force at the battle of Chiyang.

Geunchogo sacks Goguryeo’s largest city, Pyongyang, in 371 and his son Geungusu kills King Gogukwon of Goguryeo in battle.

At the end of these conquests, Baekje rings the Yellow Sea, and controls much of the Korean peninsula, including all of Gyeonggi, Chungcheong, and Jeolla and parts of Gangwon and Hwanghae provinces.