Albert Barnes Commentary Numbers 21:33

"And they turned and went up by the way of Bashan: and Og the king of Bashan went out against them, he and all his people, to battle at Edrei." — Numbers 21:33 (ASV)

In these apparently unimportant words is contained the record of the Israelite occupation of Gilead north of the Jabbok (Numbers 32:39); a territory which, though populated, like southern Gilead, by the Amorites (Deuteronomy 3:9; Joshua 2:10, and other passages), formed part of the domain of Og king of Bashan, who was himself of a different race (Deuteronomy 3:2; Joshua 12:5; Joshua 13:11). We are not told whether they were led there by express warrant of God, or whether their advance upon Bashan was provoked by Og and his people.

At Edrei - Now Edhra’ah, commonly Der’a, situated on a branch of the Jarmuk. This river formed the boundary between Gilead and Bashan.