From the middle of the river: it is not unusual, even amongst us, for a river to be divided between two lords, and for their territories or jurisdictions to meet in the middle of the river; and besides, here is a very particular reason for this expression, because the city Ar, which was no part of Sihon's dominions, but belonged to the Moabites, Deuteronomy 2:9,18, was in the middle of the river Arnon, Deuteronomy 2:36 Deuteronomy 3:16; and therefore the middle of the river is most fitly and properly here mentioned, as the bound of Sihon's dominion on that side. And from half Gilead, Heb. and the half Gilead, i.e. half of the country of Gilead: the particle from is not in the original, and this doth not seem to denote the term or bound from which his dominion begun, as our version implies, for so indeed it was not; but the place or country in and over which his dominion was, which, as is here said, began at Arnon, and took in half Gilead, and ended at Jabbok, beyond which was the other half of Gilead, which belonged to Og, as is expressly said, Joshua 12:5, where the words being wholly the same that are here, it is most reasonable to understand and translate them in the same manner.
Related Commentaries of Joshua 12:2
Joshua 12:2
2 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;