The tribes given land on the west side of the Jordan build an altar there, giving rise to a misunderstanding.
The rest of the tribes of Israel think they have set up an alternate religion, another altar for sacrifices and atonement. It is enough for them to match for war against the western tribes.
Discussions are had and an explanation given that the altar will not be used for sacrifices but as a witness that they are one people with the eastern Israelites. Everyone is happy and there is no more talk of war.
The people really get it at this point. So often they are the ones who are faithless, but here they are full of zeal for God’s kingdom.