Sihon king of the Amorites, who dwelt in Heshbon, and ruled from Aroer, which is upon the bank of the river Arnon, and from the middle of the river, and from half Gilead, even unto the river Jabbok, which is the border of the children of Ammon;
Joshua, or whoever else is the historian before he comes to sum up the new conquests Israel had made, in these verses receives their former conquests in Moses's time, under whom they became masters of the great and potent kingdoms of Sihon and Og.
Commenting on Joshua 12:1-6
Sihon king of the Amorites, who dwelt in Heshbon,.... Which he took from the Moabites, and made his capital city, Num 21:26, and ruled from Aroer, which is upon the bank of the river of Arnon; a city of Moab, which never fell into the hands of Sihon, and therefore he is said to rule from it but not over it: and from the middle...
From Aroer - Aroer was situated on the western side of the river Arnon, in the middle of the valley through which this river takes its course. The kingdom of Sihon extended from the river Arnon and the city of Aroer on the south to the river Jabbok on the north.