The tribes of Reuben, Gad, and half of Manasseh wanted to stay on the Eastside of the Jordan, but they promised to lead Israel in conquering the land on the Westside, and the Lord expected them to keep that promise,

And to the Reubenites, the Gadites, and half the tribe of Manasseh Joshua spoke, saying, “Remember the word which Moses the servant of the Lord commanded you, saying, ‘The Lord your God is giving you rest and is giving you this land.’ Your wives, your little ones, and your livestock shall remain in the land which Moses gave you on this side of the Jordan. But you shall pass before your brethren armed, all your mighty men of valor, and help them, until the Lord has given your brethren rest, as He gave you, and they also have taken possession of the land which the Lord your God is giving them. Then you shall return to the land of your possession and enjoy it, which Moses the Lord’s servant gave you on this side of the Jordan toward the sunrise” (Joshua 1.12–15).

Joshua laid out the details of the agreement that the Lord had made with the two and a half tribes so that there would be no mistaking what the Lord expected of them. They kept their promise, and so did the Lord.