The Israelites were approaching the Promised Land from the south-east. They already controlled much of the land on the east side of the Jordan River. They were now preparing to cross over the Jordan and take possession of the Promised Land.
The tribes of Reuben and Gad wanted to settle on the east side of the Jordan River because it was good land for rising their livestock. So the leaders of Reuben and Gad went to Moses and the other leaders with their request to settle on the east side of the river.
Moses became angry because these two tribes did not want to cross the river with the rest of Israel. Moses did not want to let them have their request. Moses was afraid that if their tribes did not cross over with the rest of Israel the others would not go either. Moses wanted all the tribes to cross the Jordan River together. Moses may have thought that the tribes of Reuben and Gad were afraid as their fathers were forty years earlier. Therefore, Moses, reminded the leaders of Reuben and Gad that their fathers had refused to enter the Promised Land and God punished them by not allowing them to ever enter the Promised Land. Moses did not want to see this happen again, he did not want to make God angry again at the Israelite people. So Moses refused to let them settle on the eastern side of the Jordan.
Reuben and Gad’s tribes promised that they would cross the Jordan and enter into the Promised Land with the rest of the tribes. That they were not afraid of the people who lived in Canaan. They promised to fight and defeat the people of Canaan along with the rest of Israel. But they wanted to build towns first on the east side of the Jordan and pens for their livestock. Then after they had helped defeat the Canaanites they wanted to return to the country on the eastside of the Jordan to live.
Moses agreed with the tribes of Reuben and Gad and then reminded them that if they did not keep their promise that God would punish them. They would also have to live in the land of Canaan.
Moses knew that he would die before the Israelites entered the Promised Land. (Numbers 27:12-14 So Moses gave instructions to Eleazar, Joshua and the other leaders about the agreement of the tribes of Reuben and Gad.
Each of the tribes of Reuben and Gad built towns east of the Jordan River. Half of the tribes of Manasseh had land their also. Reuben settled in the land to the south and Gad to the north of Reuben and Manasseh to the north of Gad on the eastside of the Jordan River. (Joshua 13:15-33)
Reuben and Gad kept their promise and helped the other tribes defeat the people of Canaan. Once the Israelite nation had taken possession of the Promised Land Reuben and Gad returned to the eastside of the Jordan River. (Joshua 22)