They're often also known as the "transJordan" tribes. You see, when the people of Israel were coming to the end of their 40 years of wandering in the desert and were getting ready to cross the Jordan river and enter the promised land, there were a few of the tribes who liked the look of the fertile land right where they were - on the eastern side of the river. In Numbers chapter 32, we read that the tribes of Reuben and Gad asked Moses for permission to have their territory on that side of the Jordan.
Concerned that they were perhaps unwilling to do the hard work of fighting to conquer Canaan, Moses granted them the transJordan territory on one condition: they were to cross the river with everyone else and fight alongside their brothers until the promised land was won. Reuben, Gad, and half of the tribe of Manasseh agreed to do that. All throughout the book of Joshua, they have been fighting alongside their brothers, facing the same challenges and dangers as the rest of the people.
Now, in chapter 22, the land has been conquered and it's finally time for those two and half tribes to cross the river again and go back to the place that will be their home from that time onwards. Joshua commends them for not deserting their brothers, encourages them to remain faithful to God, and sends them on their way.
One of the first things they do when they get there is to build a replica of the Lord's altar - not for making burnt offerings and sacrifices, but simply as a symbol that they are part of the same people, worshipping the same God. Because the broad Jordan river separated them from the rest of Israel, they didn't want to take the risk that some future generation would say, "You are not part of us and you have no claim to our God." Taking time to build the altar was visible evidence of their allegiance to the God of Israel.
But what happens? Their own kinsmen, who have benefitted for years from the support and solidarity of the transJordan tribes, are quick to judge them, to believe the worst of them and to accuse them of doing something terribly wrong. After such a long time of fighting side by side, the tribes on the west bank (see vs 12) now prepare to go to war against their own people.
Fortunately, despite having already judged them in their hearts, they had the good sense, or the godly wisdom, to send a delegation first - to listen to their brothers and to find out what was really going on. If only we would do that whenever we find ourselves judging and criticising others; if only we would speak directly to them, instead of constructing our own theory about the terrible thing we believe they are doing.
If speaking directly to their brothers was a good move, the way that they did it was not exactly diplomatic. They demanded to know (vs 16 - 18), "Why are you betraying the God of Israel?" and their speech was littered with accusations such as, "rebellion", "sin", and "turning away from God." The judgement in their hearts was evident in the way that they spoke. How much kinder it would have been to take an approach along the lines of, "It looks to us as if you're building an altar. Can you tell us more about what prompted you to do that?"
Fortunately, the eastern tribes responded in humility and were able to explain the true motivation for what they had done. An all-out war was avoided, and the western tribes went home in peace.
We may never completely avoid making the mistake that the western tribes made. It seems to be human nature to judge others' words or actions and forget to "believe the best" of them. But we can at least choose to respond to our critical thoughts in a godly way:
1. We can begin by confessing them to the Lord and asking for His forgiveness.
2) We can speak to the other party directly and hear their side of the story. It may be nothing at all like what we judged it to be.
3) When we do make an approach, we can be careful to use respectful and non-accusatory language, always being open to the fact that we may have judged someone completely wrongly.
If God's people would only respond in this way, we could avoid so many conflicts between individuals, so many misunderstandings between churches or groups, and so much unnecessary "going to war." May God give us the grace to deal with our judgements in a positive way.

