Jamieson, Fausset and Brown on Deuteronomy 3:12-13

12-13. this land, which we possessed at that time, from Aroer ... gave I unto the Reubenites and to the Gadites--The whole territory occupied by Sihon was parcelled out among the pastoral tribes of Reuben and Gad. It extended from the north bank of the Arnon to the south half of mount Gilead--a small mountain ridge, now called Djelaad, about six or seven miles south of the Jabbok, and eight miles in length. The northern portion of Gilead and the rich pasture lands of Bashan--a large province, consisting, with the exception of a few bleak and rocky spots, of strong and fertile soil--was assigned to the half-tribe of Manasseh.