The earliest records of Korean history are showing that approximately from the 5th century CE the Korean peninsula was raided by organized pirate groups. These pirate groups raided coastlines of Korea and controlled the marginal sea between the Japanese archipelago and Korean peninsula.
Because of the presumed origin of pirates, the inhabitants of affected areas started to name them by pejorative term Wägu 倭寇 - a combination of two Chinese characters: Wä 倭 with the original meaning dwarf, pejoratively used to call the Japanese and character Gu 寇 meaning bandit. Although the name Wägu is directing to Japanese origin of these pirate groups, their identity varied over the centuries, and it is disputable until today.
The golden era of these raids on the Korean peninsula was the second half of the 14th century after the fall of Mongol rule over the Korean kingdom of Koryŏ (高麗; 918 - 1392). Wägu periodically raided coastlines, plundered cities, kidnapped people and several times led a campaign to the capital city of the kingdom.
Together with internal political struggle and rise of the Ming dynasty in China, they were the main reason for the fall of the Koryŏ. Newly established kingdom of Chosŏn (朝鮮; 1392 - 1897) had to develop successful foreign policy and security measures to control this situation, and by the beginning of the 17th century, these raids slowly disappeared from the Korean peninsula.