Joshua 13:21

21. All the cities of the plain That is, all the other mirror cities of the southern plain which had not been particularly described.

All the kingdom of Sihon These words must not be taken as the entire extent of the territory belonging to Sihon, but must be qualified by the statement of Joshua 13:27, that the northern part of his kingdom, which extended even to the Sea of Chinnereth, was allotted to Gad. The words are therefore to be understood of the southern part of his territory, which, lying south of Mount Gilead, and including by far the larger part of the kingdom, with its capital and most important cities, might easily have been called all the kingdom.

Dukes of Sihon For their defeat by Moses see Numbers 31:1-12. These dukes were sheiks of the neighbouring towns, tributary to and dependent upon Sihon.