Adam Clarke on Joshua 12:1

From the river Arnon unto Mount Hermon - Arnon was the boundary of all the southern coast of the land occupied by the Israelites beyond Jordan; and the mountains of Hermon were the boundaries on the north. Arnon takes its rise in the mountains of Gilead, and having run a long way from north to south falls into the Dead Sea, near the same place into which Jordan discharges itself.

And all the plain on the east - All the land from the plains of Moab to Mount Hermon.