In the 14th century, several thousand pirate-warriors of Kumano launched raids on southern Kyushu. These pirate-warriors were so powerful that they were socially recognized as warrior groups just like the other powerful clans throughout Japan. Since ancient time however, the authorities, including the Shoguns, had tried to control the destructive wako.
The piracy expanded suddenly in the mid-14th century due to the civil wars. The wako of northern Kyushu and the many sea islands launched expeditions on the coast of Korea. In 1350, 100 Japanese wako ships attacked the southern coast and returned 4 more times that year. After that, the raids became more constant, and massive. Some wako fleets included as many as 350 ships in 1374 and some fleets comprised from 2,000 to 5,000 men.
The pirates mainly looted the granaries and harvests. The ease with which the pirates attacked the Korean coasts drew more warriors who were only too happy to plunder with impunity. On the open seas, pirates attacked Korean ships and also kidnapped local populations and took them back to Japan to sell as slaves or hold as ransom.
The raids became increasingly bold. Like the Vikings in earlier times, the wako sailed farther and farther up the rivers, operating as far inland as the Kaesong region near Seoul. Later, the wako began transporting horses on their ships so they could raid the interior of the country.
Other pirate chiefs turned their eyes to the riches of Ming China. After Korea, it was China's turn, in the 15th century, to suffer the bloody raids of the Vikings of the Far East.
The World Turned Upside Down: Medieval Japanese Society
The Wako: History of the Sea