Numbers 32:20-32

To preclude another Kadesh-barnea tragedy Moses commanded Reuben and Gad to go into battle with their brethren. If they would do this they could have their desired land after the fighting ended. Moses’ concern was that the whole nation should enter the land and carry out God’s will by destroying the Canaanites. If after they did this these tribes wanted to live on the other side of the Jordan, that decision would not constitute an act of disobedience to God. Transjordan was part of the Promised Land, but most of the people God commanded the Israelites to destroy lived west of the Jordan River.

Moses then instructed his successor, Joshua, in the terms of this agreement. Reuben, Gad, and half the tribe of Manasseh committed themselves to fight alongside their brethren. This is the first mention of the half tribe of Manasseh. Its leaders were apparently not as aggressive in this plan as the leaders of Reuben and Gad were.