The Wokou were also a problem encountered during King Gongmin's reign. The Wokou had been troubling the peninsula for some time and had become well-organized military marauders raiding deep into the country, rather than the "hit-and-run" bandits they started as. Generals Choi Young and Yi Seong-gye were called upon by King Gongmin to combat them. According to Korean records, wako pirates were particularly rampant roughly from 1350. After almost annual invasions of the southern provinces of Jeolla and Gyeongsang, they migrated northwards to the Chungcheong and Gyeonggi areas. The History of Goryeo has a record of sea battles in 1380 whereby one hundred warships were sent to Jinpo to rout Japanese pirates there, releasing 334 captives, Japanese sorties decreasing then after. The wako pirates were effectively expelled through the use of gunpowder technology, which the wako then lacked, after Goryeo founded the Office of Gunpowder Weapons in 1377 (but abolished twelve years later).