"And when they came to the region of the Jordan which is in the land of Canaan, the children of Reuben, the children of Gad, and half the tribe of Manasseh built an altar there by the Jordan--a great impressive altar. Now the children of Israel heard someone say, 'Behold, the children of Reuben, the children of Gad, and half the tribe of Manasseh have built an altar on the frontier of the land of Canaan, in the region of the Jordan--on the children of Israel's side.' And when the children of Israel heard of it, the whole congregation of the children of Israel gathered together at Shiloh to go to war against them" (Josh. 22:10-12).
To the uninformed, Israel's response here may seem like an overreaction. "They gathered the whole army together against these two-and-a-half tribes because they had built a remarkable altar?! What's so offensive about that?" one might ask. There is only one reason: God had given explicit instructions that all offerings were to be made at the tabernacle (cf. Lev. 17:8,9; Deut. 12:4-14). The Israelites assumed that their brethren had transgressed the covenant in a serious way (or were preparing to do so). Such an offense could not be ignored. They immediately sent a group of leaders to speak with the two-and-a-half tribes about the matter.
"What treachery is this that you have committed against the God of Israel, to turn away this day from following the LORD, in that you have built for yourselves an altar, that you might rebel this day against the LORD? Is the iniquity of Peor not enough for us, from which we are not cleansed till this day, although there was a plague in the congregation of the LORD, but that you must turn away this day from following the LORD? And it shall be, if you rebel today against the LORD, that tomorrow He will be angry with the whole congregation of Israel. Nevertheless, if the land of your possession is unclean, then cross over to the land of the possession of the LORD, where the LORD's tabernacle stands, and take possession among us; but do not rebel against the LORD, nor rebel against us, by building yourselves an altar besides the altar of the LORD our God. Did not Achan the son of Zerah commit a trespass in the accursed thing, and wrath fell on all the congregation of Israel? And that man did not perish alone in his iniquity" (Josh. 22:16-20).
This rebuke is a bit premature, as we will learn shortly, but the nation should be commended for taking the matter seriously. They assumed (incorrectly but understandably) that the altar was going to be used for sacrifices, but they knew it wasn't the authorized altar for such a purpose. They didn't want a repeat of what had happened at Peor or Ai--where the rebellion of some cost the entire nation dearly (cf. Num. 25; Josh. 7). Thus, they invite these tribes to come across the Jordan and dwell with them if their current territory is insufficient or unclean in some way. They plead with them not to sin against God and stir His anger up against all twelve tribes!
The two-and-a-half tribes give a reasonable defense for their actions. They clearly state that this altar was not for sacrifices, it was a memorial--and nothing more. One might rightly question their judgment here, but they did not sin. They had said amongst themselves - "Let us now prepare to build ourselves an altar, not for burnt offering nor for sacrifice, but that it may be a witness between you and us and our generations after us, that we may perform the service of the LORD before Him with our burnt offerings, with our sacrifices, and with our peace offerings; that your descendants may not say to our descendants in time to come, 'You have no part in the LORD.' Therefore we said that it will be, when they say this to us or to our generations in time to come, that we may say, 'Here is the replica of the altar of the LORD which our fathers made, though not for burnt offerings nor for sacrifices; but it is a witness between you and us'" (Josh. 22:26-28). The leaders brought back this message to the nation, and Israel was satisfied with this response. Dialogue helped prevent an unnecessary battle! Even today it is true that open, respectful communication will go a long way towards peacemaking.