The commanding officers report to Moshe that there was not one casualty among the Bnei Yisrael. The Tribes of Gad and Reuven, who own vast quantities of livestock, petition Moshe to remain where they are and not traverse the Jordan river into Israel. They explain that the land where they are presently located is quite suitable as grazing land for their livestock. Moshe's initial response is that this request will discourage the rest of the Bnei Yisrael, and is akin to the sin of the spies. They assure Moshe that they will first help the Bnei Yisrael wage war and conquer Israel, and only then will they go back to their homes on the eastern side of the Jordan River.
Moshe grants their request on condition that they uphold their part of the deal.
He leaves instructions with Joshua, Elazar, and other leaders regarding implementation of this transaction. The heads of the Tribes were the logical choice to explain this command to the Bnei Yisrael. They themselves served as a living example of how the power of speech can effect a change of status. After all, they originally were ordinary people, without any special status. A verbal appointment transformed them into leaders.