Though some understand this as a reference to the cities that Joshua captured by himself (Joshua 11:21, Joshua 11:23), it really just refers to the nations who have been conquered under his leadership.1 This would be all the nations west of the Jordan as opposed to the nations east of the river which were conquered by Moses.
4 Behold, I have allotted to you as an inheritance for your tribes those nations that remain, along with all the nations that I have already cut off, from the Jordan to the Great Sea in the west.