In this chapter, the Transjordan tribes return to their homes and build an altar which almost results in a civil war.
I think this is a really interesting chapter, because it brings together several different ideas that I have discussed before. To begin with, the Transjordan tribes are released from their service by Joshua, because the nation has largely captured their objectives.
The first time I read this, I was confused that they were allowed to go back because in truth, Israel has not captured the entirety of the promised land. It appears that Joshua was happy to just capture the eastern and central portions, and for whatever reason decided against invading the Philistine city-states and the northwest area around Tyre and Sidon. In fact, we previously read that Joshua told "the sons of Joseph" to conquer some of the valley lands themselves if they needed more space. So what seems to be going on is that the unified military effort is now over, and Joshua expects the remaining battles to be fought on a tribal basis. E.g. if Ephraim wants more land, they have to take it for themselves.
Under this framework, the Transjordan tribes can return to their homes because they were only obligated to remain with the national invasion to pay back the assistance they received from the other tribes that helped win the Transjordan regions.
This is all fairly straightforward. The second part of the chapter is what I find more interesting, when the Transjordan tribes build an altar on the border of the Jordan. When the other tribes hear about this, their first instinct is to think, "idolatry". They send representatives to the Transjordan tribes who say as much, referencing the "iniquity of Peor" (Num 25) and the sin of Achan (Joshua 7). In the second case, the Israelites suffered a defeat at Ai because of the sin of Achan, and that's why the nation is rising against the Transjordan tribes. If they are building an idolatrous altar, then the LORD would punish the whole nation for the sins of 2.5 tribes. The only way to avert that punishment is, like how they slew Achan, to destroy the entirety of the Transjordan tribes.
However, I think the response of the Transjordan tribes is even more interesting. What they say is that, in essence, because they live on the other side of the Jordan, the tribes inside the promised land will try to cut them off from following the LORD. This is interesting because it shows some of the subtle pitfalls for living outside of the promised land. In fact, the representatives of the nation with Phinehas even propose that the Transjordan tribes "cross into the land of the possession of the LORD", which is another way of saying, "if you find it hard to follow the LORD out here, then move in with the rest of us and let us all seek the LORD together".
This is not the first problem the 2.5 tribes have had for trying to settle outside of the promised land. When they first proposed settling there, Moses accused them of trying to discourage their brothers from invading the promised land because they were claiming their inheritance before the task was complete. So they agreed to go along the invasion even though they already claimed a land for themselves. Now they are facing another challenge, which is that the other tribes that did go into the "possession of the LORD" are likely to drift away from them socially and politically because of this natural border separating the two.
Anyway, the Transjordan tribes instead say they are building this altar as a witness between them that they will always serve the LORD. This is another OT principle which I have mentioned before, such as the "witness heap" of Jacob and Laban (Gen 31), or the various other memorials that the Israelites have constructed (Joshua 4). Like the other memorials, the Transjordan tribes built this altar to serve as a reminder that they are part of the same nation and will serve the same God.
In conclusion, I still think there is a big risk for the Transjordan tribes to separate from the tribes within the promised land, but it doesn't appear to be happening in this chapter. For now, they will remain with the nation in service to the LORD. For the future, who knows?