Ordering defectors to build ships, the Mongols began attacking the coastal islands from 1255 onward. In the Liaodong Peninsula, the Mongols eventually massed Korean defectors into a colony of 5,000 households.

Mongke Khan once again sent a large army along with Prince Yeongnyeong and Hong Bok-won, who had been taken hostage by Jalaltai as the captain, and gathered at Gapgot Daedan and showed momentum to attack Ganghwa Island. However, Kim Sugang, who had just gone to Mongolia, succeeded in persuading Mongke Khan, and the Mongols withdrew from Goryeo.