Charles Ellicott Commentary Deuteronomy 3:16-17

"And unto the Reubenites and unto the Gadites I gave from Gilead even unto the valley of the Arnon, the middle of the valley, and the border [thereof], even unto the river Jabbok, which is the border of the children of Ammon; the Arabah also, and the Jordan and the border [thereof], from Chinnereth even unto the sea of the Arabah, the Salt Sea, under the slopes of Pisgah eastward." — Deuteronomy 3:16-17 (ASV)

And unto the Reubenites and unto the Gadites I gave. —The circumstances are detailed in Numbers 32. They desired the land for their cattle.