Numbers 32 is a negotiation. Reuben and Gad, who between them own a great deal of livestock, look at the captured grazing land east of the Jordan and ask Moses to be given it rather than a share on the far side. Moses reacts with fury, telling them they are discouraging the people exactly as the spies did a generation earlier, and that their brothers will go to war while they sit down.
They propose terms. They will build folds for their flocks and towns for their families, then arm themselves and cross ahead of the rest of Israel, and not return until every tribe has its inheritance. Moses accepts, on the record and with a penalty clause, and half the tribe of Manasseh is added to the arrangement. Joshua releases them years later with a commendation for keeping it.
The problem the arrangement created is set out immediately afterwards, in Joshua 22. On the way home the eastern tribes build a large altar by the Jordan. The rest of Israel musters for war on the assumption that a second altar means a second cult, and sends a delegation first. The eastern tribes answer that the altar is not for sacrifice at all but a copy, built as a witness so that in later generations nobody can tell their children they have no share in the God of Israel because a river runs between them.
The anxiety is the point. A boundary of water was understood to be capable of becoming a boundary of religion, and the tribes on the far bank knew it before anyone else did.