John Gill Commentary Deuteronomy 2:24

"Rise ye up, take your journey, and pass over the valley of the Arnon: behold, I have given into thy hand Sihon the Amorite, king of Heshbon, and his land; begin to possess it, and contend with him in battle." — Deuteronomy 2:24 (ASV)

Rise you up, take your journey, and pass over the river Arnon ,
&c.] Which was on the border of Moab, and divided between Moab and the Amorites, (Numbers 21:13)

behold, I have given into your hand Sihon, the Amorite, king of Heshbon ,
and his land; that is, he had determined to give it to the Israelites, for as yet it was not actually given; of this king, and the place he (See Gill on Numbers 21:26) begin to possess it, and contend with him in battle; provoke him to war, fight with him, take his land from him, and enter upon the possession of it, hereby assuring of victory.