The commanding officers report to Moshe that there was not one casualty among the Bnei Yisrael. The Tribes of Gad and Reuven, who own large 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.
The Parshios of Matos and Masei, whether they are read together or apart, always fall in the days between the two fasts of the Seventeenth of Tamuz and the Ninth of Av. On the Ninth of Av, the spies brought back a distorted report about the Land of Israel, weakening the faith of the people, and bringing about forty years of wandering in the desert until all of the adult males of that generation had passed away. Throughout history, Tisha B'Av is a day ear-marked for tragedy and suffering. Moshe speaks to the Bnei Yisrael through the heads of the tribes to show how they should respect the words of their leaders. A Rabbinic decree is like a decree of the Torah, and is to be treated as the words of the Living G-d.