With peace established in Canaan, It is time to send the eastern tribes of Israel — Reuben, Gad, and part of Manasseh — back to their land across the Joran River:
1 At that time Joshua summoned the Reubenites and the Gadites and the half-tribe of Manasseh, 2 and said to them, “You have kept all that Moses the servant of the Lord commanded you and have obeyed my voice in all that I have commanded you. 3 You have not forsaken your brothers these many days, down to this day, but have been careful to keep the charge of the Lord your God. 4 And now the Lord your God has given rest to your brothers, as he promised them. Therefore turn and go to your tents in the land where your possession lies, which Moses the servant of the Lord gave you on the other side of the Jordan. 5 Only be very careful to observe the commandment and the law that Moses the servant of the Lord commanded you, to love the Lord your God, and to walk in all his ways and to keep his commandments and to cling to him and to serve him with all your heart and with all your soul.” 6 So Joshua blessed them and sent them away, and they went to their tents.
When Joshua assumed command of Israel upon the death of Moses, the second thing he does is command the people of Reuben, Gad, and Mannaseh, who have all been given land east of the Jordan River, to send their “men of valor” (גִּבּוֹרֵי הַחַיִל) across the Jordan to fight with the other tribes of Israel (10 tribes, because Manasseh has land in the middle of northern Canaan too) to take possession of the land.
When peace has come, the men of Manasseh, Gad, and Reuben will get to return to their allotments when the war is over, when all Israel has taken possession of Canaan.
Gad, Reuben, and Manasseh respond enthusiastically: “All that you have commanded us we will do, and wherever you send us we will go.”
This is loyalty. This is solidarity. This is Israel fighting together, for Gad and Reuben have no share in Canaan between the Jordan and the Great Sea itself, and Manasseh has enough of a share in the east to ignore the fight for its share in the west. They are fighting for their brothers, and not for their land.
We see something similar here when Israel fights for its newfound Canaanite allies in Gibeon.
And now that the land is at least temporarily subdued (hint: it won’t last), and there rest on all sides for Israel (interesting that Joshua does not use the word “peace” here to describe this, as the Book of Joshua does not shy away from using the word peace שָׁל֔וֹם), Joshua is fulfilling his promise to the people of Gad, Reuben, and East Manasseh. They fulfilled their obligations — they fought for the patrimony of others while theirs was already secure — and so they will be allowed to go back home to their wives and children and land with
… much wealth and with very much livestock, with silver, gold, bronze, and iron, and with much clothing. Divide the spoil of your enemies with your brothers. (Joshua 22:8 ESV)
The three eastern tribes have kept their promises, and Joshua is keeping his. Because God has kept his promises.
The only condition they have been given is to remain steadfast in their worship of Israel’s God — a command given to all Israel, not just those who are going back their homes across the river.