Deuteronomy 3:8

And we took at that time out of the hands of the two kings of
the Amorites
Sihon king of Heshbon, and Og king of Bashan;

the land that [was] on this side Jordan;
where Moses then was, being in the plains of Moab, and was the country beyond Jordan, with respect to the land of Canaan, and when in that:

from the river of Arnon unto Mount Hermon;
Arnon was a river which divided Moab and the Amorites, ( Numbers 22:13 ) and Hermon was a mountain of Gilead, which ended where Lebanon began, and was the northerly border of this country. It was remarkable for the dew that fell on it; (See Gill on Psalms 133:3).