During the Gojoseon–Yan War, which occurred in the late 4th century BC, the Yan feudal state invaded the Gojoseon kingdom.
[1] The invasion resulted in Yan's conquest of the Liaodong Peninsula from Gojoseon.
[1] Around 323 BC, Yan attempted to invade Gojoseon to strengthen the authority of its king and to obtain human and material resources.
[2] Gojoseon applied political and military pressure on Yan during the period.
[2] This escalated the tension between the two states, which would ultimately lead into the outbreak of the war.