Image: Sucker Punch Productions / PlayStationOfficial screenshot from Sucker Punch Productions / PlayStation.
A Fight for the Isle is the trophy for freeing all 24 Mongol territories in Izuhara. Six of them only become occupied once Act 2 begins.
Clear all 18 Act 1 territories, then return in Act 2 for the 6 that the Mongols retake. Aoi Village and Kechi Fishing Village are among the late ones.
Do not worry about Izuhara before Act 2. The trophy cannot pop until the last six appear.