Charles Ellicott Commentary Numbers 21:26

"For Heshbon was the city of Sihon the king of the Amorites, who had fought against the former king of Moab, and taken all his land out of his hand, even unto the Arnon." — Numbers 21:26 (ASV)

And taken all his land ... — that is, the land between the Arnon and the Jabbok, as it is explained in the last clause of the verse.