Later in Joshua 22 we are told that the western tribes interpreted the altar as a breach of faith and rebellion against the Lord. The reason for this is that the Lord had commanded his people to only offer sacrifices at the tent of meeting (Leviticus 17:1–9; Deuteronomy 12:1–32). The western tribes think that the eastern tribes are no longer interested in serving the Lord and that their actions are equivalent to apostasy. As a result, in keeping with the conditions of Deuteronomy 13:12–18, the people gather for war and send a delegation to make inquiries. The reaction of the western tribes is thus not rash or hot-headed, it is in keeping with what God commanded.
11 And the people of Israel heard it said, “Behold, the people of Reuben and the people of Gad and the half-tribe of Manasseh have built the altar at the frontier of the land of Canaan, in the region about the Jordan, on the side that belongs to the people of Israel.”